Category: Fishworks

When the Sun Storage 7000 was first introduced, a key design decision was to allow only a single ZFS storage pool per host. This forces users to fully take advantage of the ZFS pool storage model, and prevents them from adopting ludicrous schemes such as “one pool per filesystem.” While RAID-Z has non-trivial performance implications for IOPs-bound workloads, the hope was that by allowing logzilla and readzilla devices to be configured per-filesystem, users could adjust relative performance and implement different qualities of service on a single pool.

While this works for the majority of workloads, there are still some that benefit from mirrored performance even in the presence of cache and log devices. As the maximum size of Sun Storage 7000 systems increases, it became apparent that we needed a way to allow pools with different RAS and performance characteristics in the same system. With this in mind, we relaxed the “one pool per system” rule¹ with the 2010.Q1 release.

The storage configuration user experience is relatively unchanged. Instead of having a single pool (or two pools in a cluster), and being able to configure one or the other, you can simply click the ‘+’ button and add pools as needed. When creating a pool, you can now specify a name for the pool. When importing a pool, you can either accept the existing name or give it a new one at the time you select the pool. Ownership of pools in a cluster is now managed exclusively through the Configuration -> Cluster screen, as with other shared resources.

When managing shares, there is a new dropdown menu at the top left of the navigation bar. This controls which shares are shown in the UI. In the CLI, the equivalent setting is the ‘pool’ property at the ‘shares’ node.

While this gives some flexibility in storage configuration, it also allows users to create poorly constructed storage topologies. The intent is to allow the user to create pools with different RAS and performance characteristics, not to create dozens of different pools with the same properties. If you attempt to do this, the UI will present a warning summarizing the drawbacks if you were to continue:

• Wastes system resources that could be shared in a single pool.
• Decreases overall performance
• Increases administrative complexity.
• Log and cache devices can be enabled on a per-share basis.

You can still commit the operation, but such configurations are discouraged. The exception is when configuring a second pool on one head in a cluster.

We hope this feature will allow users to continue to consolidate storage and expand use of the Sun Storage 7000 series in more complicated environments.

1. Clever users figured out that this mechanism could be circumvented in a cluster to have two pools active on the same host in an active/passive configuration.

In my previous entry, I described the overall architecture of shadow migration. This post will dive into the details of how it’s actually implemented, and the motivation behind some of the original design decisions.

VFS interposition

A very early desire was that we wanted something that could migrate data from many different sources. And while ZFS is the primary filesystem for Solaris, we also wanted to allow for arbitrary local targets. For this reason, the shadow migration infrastructure is implemented entirely at the VFS (Virtual FileSystem) layer. At the kernel level, there is a new ‘shadow’ mountpoint option, which is the path to another filesystem on the system. The kernel has no notion of whether a source filesystem is local or remote, and doesn’t differentiate between synchronous access and background migration. Any filesystem access, whether it is local or over some other protocol (CIFS, NFS, etc) will use the VFS interfaces and therefore be fed through our interposition layer.

The only other work the kernel does when mounting a shadow filesystem is check to see if the root directory is empty. If it is empty, we create a blank SUNWshadow extended attribute on the root directory. Once set, this will trigger all subsequent migration as long as the filesystem is always mounted with the ‘shadow’ attribute. Each VFS operation first checks to see if the filesystem is shadowing another (a quick check), and then whether the file or directory has the SUNWshadow attribute set (slightly more expensive, but cached with the vnode). If the attribute isn’t present, then we fall through to the local filesystem. Otherwise, we migrate the file or directory and then fall through to the local filesystem.

Migration of directories

In order to migrate a directory, we have to migrate all the entriest. When migrating an entry for a file, we don’t want to migrate the complete contents until the file is accessed, but we do need to migrate enough metadata such that access control can be enforced. We start by opening the directory on the remote side whose relative path is indicated by the SUNWshadow attribute. For each directory entry, we create a sparse file with the appropriate ownership, ACLs, system attributes and extended attributes.

Once the entry has been migrated, we then set a SUNWshadow attribute that is the same as the parent but with “/” appended where “name” is the directory entry name. This attribute always represents the relative path of the unmigrated entity on the source. This allows files and directories to be arbitrarily renamed without losing track of where they are located on the source. It also allows the source to change (i.e. restored to a different host) if needed. Note that there are also types of files (symlinks, devices, etc) that do not have contents, in which case we simply migrate the entire object at once.

Once the diretory has been completely migrated, we remove the SUNWshadow attribute so that future accesses all use the native filesystem. If the process is interrupted (system reset, etc), then the attribute will still be on the parent directory so we will migrate it again when the user (or background process) tries to access it.

Migration of files

Migrating a plain file is much simpler. We use the SUNWshadow attribute to locate the file on the source, and then read the source file and write the corresponding data to the local filesystem. In the current software version, this happens all at once, meaning the first access of a large file will have to wait for the entire file to be migrated. Future software versions will remove this limitation and migrate only enough data to satisfy the request, as well as allowing concurrent accesses to the file. Once all the data is migrated, we remove the SUNWshadow attribute and future accesses will go straight to the local filesystem.

Dealing with hard links

One issue we knew we’d have to deal with is hard links. Migrating a hard link requires that the same file reference appear in multiple locations within the filesystem. Obviously, we do not know every reference to a file in the source filesystem, so we need to migrate these links as we discover them. To do this, we have a special directory in the root of the filesystem where we can create files named by their source FID. A FID (file ID) is a unique identifier for the file within the filesystem. We create the file in this hard link directory with a name derived from its FID. Each time we encounter a file with a link count greater than 1, we lookup the source FID in our special directory. If it exists, we create a link to the directory entry instead of migrating a new instance of the file. This way, it doesn’t matter if files are moved around, removed from the local filesystem, or additional links created. We can always recreate a link to the original file. The one wrinkle is that we can migrate from nested source filesystems, so we also need to track the FSID (filesystem ID) which, while not persistent, can be stored in a table and reconstructed using source path information.

Completing migration

A key feature of the shadow migration design is that it treats all accesses the same, and allows background migration of data to be driven from userland, where it’s easier to control policy. The downside is that we need the ability to know when we have finished migrating every single file and directory on the source. Because the local filesystem is actively being modified while we are traversing, it’s impossible to know whether you’ve visited every object based only on walking the directory hierarchy. To address this, we keep track of a “pending” list of files and directories with shadow attributes. Every object with a shadow attribute must be present in this list, though this list can contain objects without shadow attributes, or non-existant objects. This allows us to be synchronous when it counts (appending entries) and lazy when it doesn’t (rewriting file with entries removed). Most of the time, we’ll find all the objects during traversal, and the pending list will contain no entries at all. In the case we missed an object, we can issue an ioctl() to the kernel to do the work for us. When that list is empty we know that we are finished and can remove the shadow setting.

ZFS integration

The ability to specify the shadow mount option for arbitrary filesystems is useful, but is also difficult to manage. It must be specified as an absolute path, meaning that the remote source of the mount must be tracked elsewhere, and has to be mounted before the filesystem itself. To make this easier, a new ‘shadow’ property was added for ZFS filesystems. This can be set using an abstract URI syntax (“nfs://host/path”), and libzfs will take care of automatically mounting the shadow filesystem and passing the correct absolute path to the kernel. This way, the user can manage a semantically meaningful relationship without worrying about how the internal mechanisms are connected. It also allows us to expand the set of possible sources in the future in a compatible fashion.

Hopefully this provides a reasonable view into how exactly shadow migration works, and the design decisions behind it. The goal is to eventually have this available in Solaris, at which point all the gritty details should be available to the curious.

When ZFS was first developed, the engineering team had the notion that pooled storage would make filesystems cheap and plentiful, and we’d move away from the days of /export1, /export2, ad infinitum. From the ZFS perspective, they are cheap. It’s very easy to create dozens or hundreds of filesystems, each which functions as an administrative control point for various properties. However, we quickly found that other parts of the system start to break down once you get past 1,000 or 10,000 filesystems. Mounting and sharing filesystems takes longer, browsing datasets takes longer, and managing automount maps (for those without NFSv4 mirror mounts) quickly spirals out of control.

For most users this isn’t a problem – a few hundred filesystems is more than enough to manage disparate projects and groups on a single machine. There was one class of users, however, where a few hundred filesystems wasn’t enough. These users were university or other home directory environments with 20,000 or more users, each which needed to have a quota to guarantee that they couldn’t run amok on the system. The traditional ZFS solution, creating a filesystem for each user and assigning a quota, didn’t scale. After thinking about it for a while, Matt developed a fairly simple architecture to provide this functionality without introducing pathological complexity into the bowels of ZFS. In build 114 of Solaris Nevada, he pushed the following:

PSARC 2009/204 ZFS user/group quotas & space accounting

This provides full support for user and group quotas on ZFS, as well as the ability to track usage on a per-user or per-group basis within a dataset.

This was later integrated into the 2009.Q3 software release, with an additional UI layer. From the ‘general’ tab of a share, you can query usage and set quotas for individual users or groups quickly. The CLI allows for automated batch operations. Requesting a single user or group is significantly faster than requesting all the current usage, but you an also get a list of the current usage for a project or share. With integrated identity management, users and groups can be specified either by UNIX username or Windows name.

There are some significant differences between user and group quotas and traditional ZFS quotas. The following is an excerpt from the on-line documentation on the subject:

• User and group quotas can only be applied to filesystems.
• User and group quotas are implemented using delayed enforcement. This means that users will be able to exceed their quota for a short period of time before data is written to disk. Once the data has been pushed to disk, the user will receive an error on new writes, just as with the filesystem-level quota case.
• User and group quotas are always enforced against referenced data. This means that snapshots do not affect any quotas, and a clone of a snapshot will consume the same amount of effective quota, even though the underlying blocks are shared.
• User and group reservations are not supported.
• User and group quotas, unlike data quotas, are stored with the regular filesystem data. This means that if the filesystem is out of space, you will not be able to make changes to user and group quotas. You must first make additional space available before modifying user and group quotas.
• User and group quotas are sent as part of any remote replication. It is up to the administrator to ensure that the name service environments are identical on the source and destination.
• NDMP backup and restore of an entire share will include any user or group quotas. Restores into an existing share will not affect any current quotas. (There is currently a bug preventing this from working in the initial release, which will be fixed in a subsequent minor release.)

This feature will hopefully allow the Sun Storage 7000 series to function in environments where it was previously impractical to do so. Of course, the real person to thank is Matt and the ZFS team – it was a very small amount of work to provide an interface on top of the underlying ZFS infrastructure.

In the Sun Storage 7000 2009.Q3 software release, one of the major new features I worked on was the addition of what we termed “shadow migration.” When we launched the product, there was no integrated way to migrate data from existing systems to the new systems. This resulted in customers rolling it by hand (rsync, tar, etc), or paying for professional services to do the work for them. We felt that we could present a superior model that would provide for a more integrated experience as well as let the customer leverage the investment in the system even before the migration was complete.

The idea in and of itself is not new, and various prototypes of this have been kicking around inside of Sun under various monikers (“brain slug”, “hoover”, etc) without ever becoming a complete product. When Adam and myself sat down shortly before the initial launch of the product, we decide we could do this without too much work by integrating the functionality directly in the kernel. The basic design requirements we had were:

• We must be able to migrate over standard data protocols (NFS) from arbitrary data sources without the need to have special software running on the source system.
• Migrated data must be available before the entire migration is complete, and must be accessible with native performance.
• All the data to migrate the filesystem must be stored within the filesystem itself, and must not rely on an external database to ensure consistency.

With these requirements in hand, our key insight was that we could create a “shadow” filesystem that could pull data from the original source if necessary, but then fall through to the native filesystem for reads and writes one the file has been migrated. What’s more, we could leverage the NFS client on Solaris and do this entirely at the VFS (virtual filesystem) layer, allowing us to migrate data between shares locally or (eventually) over other protocols as well without changing the interpositioning layer. The other nice thing about this architecture is that the kernel remains ignorant of the larger migration process. Both synchronous requests (from clients) and background requests (from the management daemon) appear the same. This allows us to control policy within the userland software stack, without pushing that complexity into the kernel. It also allows us to write a very comprehensive automated test suite that runs entirely on local filesystems without need a complex multi-system environment.

So what’s better (and worse) about shadow migration compared to other migration strategies? For that, I’ll defer to the documentation, which I’ve reproduced here for those who don’t have a (virtual) system available to run the 2009.Q3 release:

Migration via synchronization

This method works by taking an active host X and migrating data to the new host Y while X remains active. Clients still read and write to the original host while this migration is underway. Once the data is initially migrated, incremental changes are repeatedly sent until the delta is small enough to be sent within a single downtime window. At this point the original share is made read-only, the final delta is sent to the new host, and all clients are updated to point to the new location. The most common way of accomplishing this is through the rsync tool, though other integrated tools exist. This mechanism has several drawbacks:

• The anticipated downtime, while small, is not easily quantified. If a user commits a large amount of change immediately before the scheduled downtime, this can increase the downtime window.
• During migration, the new server is idle. Since new servers typically come with new features or performance improvements, this represents a waste of resources during a potentially long migration period.
• Coordinating across multiple filesystems is burdensome. When migrating dozens or hundreds of filesystems, each migration will take a different amount of time, and downtime will have to be scheduled across the union of all filesystems.

Migration via external interposition

This method works by taking an active host X and inserting a new appliance M that migrates data to a new host Y. All clients are updated at once to point to M, and data is automatically migrated in the background. This provides more flexibility in migration options (for example, being able to migrate to a new server in the future without downtime), and leverages the new server for already migrated data, but also has significant drawbacks:

• The migration appliance represents a new physical machine, with associated costs (initial investment, support costs, power and cooling) and additional management overhead.
• The migration appliance represents a new point of failure within the system.
• The migration appliance interposes on already migrated data, incurring extra latency, often permanently. These appliances are typically left in place, though it would be possible to schedule another downtime window and decommission the migration appliance.

Shadow migration

Shadow migration uses interposition, but is integrated into the appliance and doesn’t require a separate physical machine. When shares are created, they can optionally “shadow” an existing directory, either locally (see below) or over NFS. In this scenario, downtime is scheduled once where the source appliance X is placed into read-only mode, a share is created with the shadow property set, and clients are updated to point to the new share on the Sun Storage 7000 appliance. Clients can then access the appliance in read-write mode.

Once the shadow property is set, data is transparently migrated in the background from the source appliance locally. If a request comes from a client for a file that has not yet been migrated, the appliance will automatically migrate this file to the local server before responding to the request. This may incur some initial latency for some client requests, but once a file has been migrated all accesses are local to the appliance and have native performance. It is often the case that the current working set for a filesystem is much smaller than the total size, so once this working set has been migrated, regardless of the total native size on the source, there will be no perceived impact on performance.

The downside to shadow migration is that it requires a commitment before the data has finished migrating, though this is the case with any interposition method. During the migration, portions of the data exists in two locations, which means that backups are more complicated, and snapshots may be incomplete and/or exist only on one host. Because of this, it is extremely important that any migration between two hosts first be tested thoroughly to make sure that identity management and access controls are setup correctly. This need not test the entire data migration, but it should be verified that files or directories that are not world readable are migrated correctly, ACLs (if any) are preserved, and identities are properly represented on the new system.

Shadow migration implemented using on-disk data within the filesystem, so there is no external database and no data stored locally outside the storage pool. If a pool is failed over in a cluster, or both system disks fail and a new head node is required, all data necessary to continue shadow migration without interruption will be kept with the storage pool.

In a subsequent post, I’ll discuss some of the thorny implementation detail we had to solve, as well as provide some screenshots of migration in progress. In the meantime, I suggest folks download the simulator and upgrade to the latest software to give it a try.

Last week, we announced the Sun Storage 7310 system. At the same time, a less significant but still notable change was made to the Sun Storage product line. The Sun Storage 7210 system is now expandable via J4500 JBODs. These JBODs have the same form factor as the 7210 – 48 drives in a top loading 4u form factor. Up to two J4500s can be added to a 7210 system via a single HBA, resulting in up to 142 TB of storage in 12 RU of space. JBODs with 500G or 1T drives are supported.

For customers looking for maximum density without high availability, the combination of 7210 with J4500 provides a perfect solution.

In the past, I’ve discussed the evolution of disk FMA. Much has been accomplished in the past year, but there are still several gaps when it comes to ZFS and disk faults. In Solaris today, a fault diagnosed by ZFS (a device failing to open, too many I/O errors, etc) is reported as a pool name and 64-bit vdev GUID. This description leaves something to be desired, referring the user to run zpool status to determine exactly what went wrong. But the user is still has to know how to go from a cXtYdZ Solaris device name to a physical device, and when they do locate the physical device they need to manually issue a zpool replace command to initiate the replacement.

While this is annoying in the Solaris world, it’s completely untenable in an appliance environment, where everything needs to “just work”. With that in mind, I set about to plug the last few holes in the unified plan:

• ZFS faults must be associated with a physical disk, including the human-readable label
• A disk fault (ZFS or SMART failure) must turn on the associated fault LED
• Removing a disk (faulted or otherwise) and replacing it with a new disk must automatically trigger a replacement

While these seem like straightforward tasks, as usual they are quite difficult to get right in a truly generic fashion. And for an appliance, there can be no Solaris commands or additional steps for the user. To start with, I needed to push the FRU information (expressed as a FMRI in the hc libtopo scheme) into the kernel (and onto the ZFS disk label) where it would be available with each vdev. While it is possible to do this correlation after the fact, it simplifies the diagnosis engine and is required for automatic device replacement. There are some edge conditions around moving and swapping disks, but was relatively straightforward. Once the FMRI was there, I could include the FRU in the fault suspect list, and using Mike’s enhancements to libfmd_msg, dynamically insert the FRU label into the fault message. Traditional FMA libtopo labels do not include the chassis label, so in the Fishworks stack we go one step further and re-write the label on receipt of a fault event with the user-defined chassis name as well as the physical slot. This message is then used when posting alerts and on the problems page. We can also link to the physical device from the problems page, and highlight the faulty disk in the hardware view.

With the FMA plumbing now straightened out, I needed a way to light the fault LED for a disk, regardless of whether it was in the system chassis or an external enclosure. Thanks to Rob’s sensor work, libtopo already presents a FMRI-centric view of indicators in a platform agnostic manner. So I rewrote the disk-monitor module (or really, deleted everything and created a new fru-monitor module) that would both poll for FRU hotplug events, as well as manage the fault LEDs for components. When a fault is generated, the FRU monitor looks through the suspect list, and turns on the fault LED for any component that has a supported indicator. This is then turned off when the corresponding repair event is generated. This also had the side benefit of generating hotplug events phrased in terms of physical devices, which the appliance kit can use to easily present informative messages to the user.

Finally, I needed to get disk replacement to work like everyone expects it to: remove a faulted disk, put in a new one, and walk away. The genesis of this functionality was putback to ON long ago as the autoreplace pool property. In Solaris, this functionality only works with disks that have static device paths (namely SATA). In the world of multipathed SAS devices, the device path is really a scshi_vhci node identified by the device WWN. If we remove a disk and insert a new one, it will appear as a new device with no way to correlate it to the previous instance, preventing us from replacing the correct vdev. What we need is physical slot information, which happens to be provided by the FMRI we are already storing with the vdev for FMA purposes. When we receive a sysevent for a new device addition, we look at the latest libtopo snapshot and take the FMRI of the newly inserted device. By looking at the current vdev FRU information, we can then associate this with the vdev that was previously in the slot, and automatically trigger the replacement.

This process took a lot longer than I would have hoped, and has many more subtleties too boring even for a technical blog entry, but it is nice to sit back and see a user experience that is intuitive, informative, and straightforward – the hallmarks of an integrated appliance solution.