SVM and the B_FAILFAST flag

Now that
OpenSolaris
is here it is a lot easier to talk about some of the interesting
implementation details in the code. In this post I wanted to
discuss the first project I did after I started to work on the
Solaris Volume Manager (SVM). This is on my mind right now
because it also happens to be related to one of my most recent changes
to the code. This change is not even in
Solaris Express
yet, it is only available in
OpenSolaris.
Early access to these kind of changes is just one small reasons why OpenSolaris is so cool.

My first SVM project was to add support for the B_FAILFAST flag.
This flag is defined in /usr/include/sys/buf.h and it was
implemented in some of the disk drivers so that I/O requests
that were queued in the driver could be cleared out quickly when
the driver saw that a disk was malfunctioning. For SVM the
big requester for this feature was our clustering software. The
problem they were seeing was that in a production environment
there would be many concurrent I/O requests queued up down in
the sd driver. When the disk was failing the sd driver would
need to process each of these requests, wait for the timeouts
and retrys and slowly drain its queue. The cluster software
could not failover to another node until all of these pending
requests had been cleared out of the system. The B_FAILFAST flag
is the exact solution to this problem. It tells the driver
to do two things. First, it reduces the number of retries that
the driver does to a failing disk before it gives up and returns
an error. Second, when the first I/O buf that is queued up in
the driver gets an error, the driver will immediately error
out all of the other, pending bufs in its queue. Furthermore,
any new bufs sent down with the B_FAILFAST flag will immediately
return with an error.

This seemed fairly straightforward to implement in SVM. The
code had to be modified to detect if the underlying devices
supported the B_FAILFAST flag and if so, the flag should be
set in the buf that was being passed down from the md driver
to the underlying drivers that made up the metadevice. For
simplicity we decided we would only add this support to the
mirror metadevice in SVM. However, the more I looked at this,
the more complicated it seemed to be. We were worried about
creating new failure modes with B_FAILFAST and the big concern was the
possibility of a “spurious” error. That is, getting back an
error on the buf that we would not have seen if we had let the
underlying driver perform its full set of timeouts and retries.
This concern eventually drove the whole design of the initial B_FAILFAST
implementation within the mirror code. To handle this spurious
error case I implemented an algorithm within the driver so that when we
got back an errored B_FAILFAST buf we would resubmit that buf without the
B_FAILFAST flag set. During this retry, all of the other failed I/O bufs would
also immediately come back up into the md driver. I queued those
up so that I could either fail all of the them after the retried buf
finally failed or I could resubmit them back down to the underlying
driver if the retried I/O succeeded. Implementing this correctly
took a lot longer than I originally expected when I took this first
project and it was one of those things that worked but I was never
very happy with. The code was complex and I never felt
completely confident that there wasn’t some obscure error condition
lurking here that would come back to bite us later. In addition,
because of the retry, the failover of a component within a mirror
actually took *longer* now if there was only a single I/O
being processed.

This original algorithm was delivered in the S10 code and was also released
as a patch for S9 and SDS 4.2.1. It has been in use for a couple of years
which gave me some direct experience with how well the B_FAILFAST
option worked in real life. We actually have seen one or two
of these so called spurious errors but in all cases there were real,
underlying problems with the disks. The storage was marginal
and SVM would have been better off just erroring out those components
within the mirror and immediately failing over to the good side
of the mirror. By this time I was comfortable with this idea so
I rewrote the B_FAILFAST code within the mirror driver. This new
algorithm is what you’ll see today in the
OpenSolaris
code base. I basically decided to just trust the error we get
back when B_FAILFAST is set. The code will follow the normal error
path so that it puts the submirror component into the maintenance state
and just uses the other, good side of the mirror from that point onward.
I was able to remove the queue and simplify the logic almost back to
what it was before we added support for B_FAILFAST.

However, there is still one special case we have to worry about when
using B_FAILFAST. As I mentioned above, when B_FAILFAST is set, all
of the pending I/O bufs that are queued down in the underlying driver
will fail once the first buf gets an error. When we are down to the
last side of a mirror the SVM code will continue to try to do I/O to
the those last submirror components, even though they are taking errors.
This is called the LAST_ERRED state within SVM and is an attempt to try to
provide access to as much of your data as possible. When using B_FAILFAST
it is probable that not all of the failed I/O bufs will have been
seen by the disk and given a chance to succeed. With the new algorithm
the code detects this state and reissues all of the I/O bufs without
B_FAILFAST set. There is no longer any queueing, we just resubmit the I/O
bufs without the flag and all future I/O to the submirror is done
without the flag. Once the LAST_ERRED state is cleared the code will
return to using the B_FAILFAST flag.

All of this is really an implementation detail of mirroring in SVM.
There is no user-visible component of this except for a change in
the behavior of how quickly the mirror will fail the errored drives
in the submirror. All of the code is contained within the mirror
portion of the SVM driver and you can see it in
mirror.c.
The function
mirror_check_failfast
is used to determine if all of the components
in a submirror support using the B_FAILFAST flag. The
mirror_done
function is called when the I/O to the underlying submirror is complete.
In this function we check if the I/O failed and if B_FAILFAST was set.
If so we call the
submirror_is_lasterred
function to check for that
condition and the
last_err_retry
function is called only when we
need to resubmit the I/O. This function is actually executed in
a helper thread since the I/O completes in a thread separately from
the thread that initiated the I/O down into the md driver.

To wrap up, the SVM md driver code lives in the source tree at
usr/src/uts/common/io/lvm.
The main md driver is in the
md
subdirectory
and each specific kind of metadevice also has its own subdirectory (
mirror,
stripe,
etc.). The SVM command line utilities live in
usr/src/cmd/lvm
and the shared library code that SVM uses lives in
usr/src/lib/lvm.
Libmeta
is the primary library. In another post
I’ll talk in more detail about some of these other components of SVM.

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