On 12/02/2015 01:41 AM, Dave Chinner wrote:
On Tue, Dec 01, 2015 at 10:56:01PM +0200, Avi Kivity wrote:
On 12/01/2015 10:45 PM, Dave Chinner wrote:
On Tue, Dec 01, 2015 at 09:01:13AM -0500, Glauber Costa wrote:
The difference is an allocation can block waiting on IO, and the
CPU can then go off and run another process, which then tries to do
an allocation. So you might only have 4 CPUs, but a workload that
can have a hundred active allocations at once (not uncommon in
file server workloads).
But for us, probably not much more. We try to restrict active I/Os
to the effective disk queue depth (more than that and they just turn
sour waiting in the disk queue).
On worklaods that are roughly 1 process per CPU, it's typical that
agcount = 2 * N cpus gives pretty good results on large filesystems.
This is probably using sync calls. Using async calls you can have
many more I/Os in progress (but still limited by effective disk
queue depth).
Ah, no. Even with async IO you don't want unbound allocation
concurrency.
Unbound, certainly not.
But if my disk want 100 concurrent operations to deliver maximum
bandwidth, and XFS wants fewer concurrent allocations to satisfy some
internal constraint, then I can't satisfy both.
To be fair, the number 100 was measured for 4k reads. It's sure to be
much lower for 128k writes, and since we set an extent size hint of 1MB,
only 1/8th of those will be allocating. So I expect things to work in
practice, at least with the current generation of disks. Unfortunately
disk bandwidth is growing faster than latency is improving, which means
that the effective concurrency is increasing.
The allocation algorithms rely on having contiguous
free space extents that are much larger than the allocations being
done to work effeectively and minimise file fragmentation. If you
chop the filesystem up into lots of small AGs, then it accelerates
the rate at which the free space gets chopped up into smaller
extents and performance then suffers. It's the same problem as
running a large filesystem near ENOSPC for an extended period of
time, which again is something we most definitely don't recommend
you do in production systems.
I understand. I guess it makes ag randomization even more important,
for our use case.
What happens when an ag fills up? Can a file overflow to another ag?
If you've got 400GB filesystems or you are using spinning disks,
then you probably don't want to go above 16 AGs, because then you
have problems with maintaining continugous free space and you'll
seek the spinning disks to death....
We're concentrating on SSDs for now.
Sure, so "problems with maintaining continugous free space" is what
you need to be concerned about.
Right. Luckily our allocation patterns are very friendly towards that.
We have append-only files that grow rapidly, then are immutable for a
time, then are deleted. (It is a log-structured database so a natual fit
for SSDs).
We can increase our extent size hint if it will help the SSD any.
'mount -o ikeep,'
Interesting. Our files are large so we could try this.
Keep in mind that ikeep means that inode allocation permanently
fragments free space, which can affect how large files are allocated
once you truncate/rm the original files.
We can try to prime this by allocating a lot of inodes up front,
then removing them, so that this doesn't happen.
Again - what problem have you measured that inode preallocation will
solves in your application? Don't make changes just because you
*think* it will fix what you *think* is a problem. Measure, analyse,
solve, in that order.
We are now investigating what we can do to fix the problem, we aren't
committing to any solution yet. Certainly we plan to be certain of what
the problem is before we fix it.
Up until a few days ago we never saw any blocks with XFS, and were very
happy -- but that was with 90us, 450k IOPS disks. With the slower
disks, accessed through a certain hypervisor, we do see XFS block, and
it is very worrying.
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