On 12/04/2015 05:16 AM, Dave Chinner wrote:
On Thu, Dec 03, 2015 at 02:52:08PM +0200, Avi Kivity wrote:
On 12/03/2015 01:19 AM, Dave Chinner wrote:
On Wed, Dec 02, 2015 at 11:02:08AM +0200, Avi Kivity wrote:
On 12/02/2015 01:06 AM, Dave Chinner wrote:
On Tue, Dec 01, 2015 at 11:38:29PM +0200, Avi Kivity wrote:
On 12/01/2015 11:19 PM, Dave Chinner wrote:
XFS spread files across the allocation groups, based on the directory these
files are created,
Idea: create the files in some subdirectory, and immediately move
them to their required location.
....
My hack involves creating the file in a random directory, and while
it is still zero sized, move it to its final directory. This is
simply to defeat the ag selection heuristic.
Which you really don't want to do.
Why not? For my directory structure, files in the same directory do
not share temporal locality. What does the ag selection heuristic
give me?
Wrong question. The right question is this: what problems does
subverting the AG selection heuristic cause me?
If you can't answer that question, then you can't quantify the risks
involved with making such a behavioural change.
Okay. Any hint about the answer to that question?
If your file set is randomly distributed across the filesystem,
I think that happens whether or not I break the "files in the same
directory are related" heuristic, because I have many directories. It's
just that some of them get churned more than others.
then
it's quite likely that the filesystem will use all of the LBA space
rather than reusing the same AGs and hence LBA regions. That's going
to slowly fragment free space as metadata (which has different
lifetimes to data) and long term data gets more widely distributed.
That, in term will slowly result in the working dataset being made
up of more and smaller extents, whcih will also slowly get more
distributed over time, which them means allocation and freeing of
extents takes longer, trim becomes less effective because it's
workingwith smaller spaces, the SSD's "LBA in use" mapping becomes
more fragmented so garbage collection becomes harder, etc...
But, really, the only way to tell is to test, measure, observe and
analyse....
Sure.
This is pointless for an SSD. Perhaps XFS should randomize the ag on
nonrotational media instead.
Actually, no, it is not pointless. SSDs do not require optimisation
for minimal seek time, but data locality is still just as important
as spinning disks, if not moreso. Why? Because the garbage
collection routines in the SSDs are all about locality and we can't
drive garbage collection effectively via discard operations if the
filesystem is not keeping temporally related files close together in
it's block address space.
In my case, files in the same directory are not temporally related.
But I understand where the heuristic comes from.
Maybe an ioctl to set a directory attribute "the files in this
directory are not temporally related"?
And exactly what does that gain us?
I have a directory with commitlog files that are constantly and
rapidly being created, appended to, and removed, from all logical
cores in the system. Does this not put pressure on that allocation
group's locks?
Not usually, because if an AG is contended, the allocation algorithm
skips the contended AG and selects the next uncontended AG to
allocate in. And given that the append algorithm used by the
allocator attempts to use the last block of the last extent as the
target for the new extent (i.e. contiguous allocation) once a file
has skipped to a different AG all allocations will continue in that
new AG until it is either full or it becomes contended....
IOWs, when AG contention occurs, the filesystem automatically
spreads out the load over multiple AGs. Put simply, we optimise for
locality first, but we're willing to compromise on locality to
minimise contention when it occurs. But, also, keep in mind that
in minimising contention we are still selecting the most local of
possible alternatives, and that's something you can't do in
userspace....
Cool. I don't think "nearly-local" matters much for an SSD (it's
either contiguous or it is not), but it's good to know that it's
self-tuning wrt. contention.
"Nearly local" matters a lot for filesystem free space management
and hence minimising the amount o LBA space the filesystem actually
uses in the long term given a relatively predicatable workload....
In some good news, Glauber hacked our I/O engine not to throw so
many concurrent I/Os at the filesystem, and indeed so the contention
reduced. So it's likely we were pushing the fs so hard all the ags
were contended, but this is no longer the case.
What is the xfs_info output of the filesystem you tested on?
It was a cloud disk so someone else now has the pleasure...
With the way the XFS allocator works, it fills AGs from lowest to
highest blocks, and if you free lots of space down low in the AG
then that tends to get reused before the higher offset free space.
hence the XFS allocates space in the above workload would result in
roughly 1/3rd of the LBA space associated with the filesystem
remaining unused. This is another allocator behaviour designed for
spinning disks (to keep the data on the faster outer edges of
drives) that maps very well to internal SSD allocation/reclaim
algorithms....
Cool. So we'll keep fstrim usage to daily, or something similarly low.
Well, it's something you'll need to monitor to determine what the
best frequency is, as even fstrim doesn't come for free (esp. if the
storage does not support queued TRIM commands).
I was able to trigger a load where discard caused io_submit to sleep
even on my super-fast nvme drive.
The bad news is, disabling discard and running fstrim in parallel with
this load also caused io_submit to sleep.
FWIW, did you know that TRIM generally doesn't return the disk to
the performance of a pristine, empty disk? Generally only a secure
erase will guarantee that a SSD returns to "empty disk" performance,
but that also removes all data from then entire SSD. Hence the
baseline "sustained performance" you should be using is not "empty
disk" performance, but the performance once the disk has been
overwritten completely at least once. Only them will you tend to see
what effect TRIM will actually have.
I did not know that. Maybe that's another factor in why cloud SSDs
are so slow.
Have a look at the random write performance consistency graphs for
the different enterprise SSDs here:
http://www.anandtech.com/show/9430/micron-m510dc-480gb-enterprise-sata-ssd-review/3
You'll see just how different sustained write load performance is to
the empty drive performance (which is only the first few hundred
seconds of each graph) across the different drives that have been
tested. The next page has similar results for mixed random
read/write workloads....
That will give you a good idea of how the current enterprise SSDs
behave under sustained write load. It's a *lot* better than the way
the 1st and 2nd generation drives performed....
write 10%-20% of the disk's capacity.
Run the workload to steady state performance and measure the
degradation as it continues to run and overwrite the SSDs
repeatedly. To do this properly you are going to have to sacrifice
some SSDs, because you're going to need to overwrite them quite a
few times to get an idea of the degradation characteristics and
whether a periodic trim makes any difference or not.
Enterprise SSDs are guaranteed for something like N full writes /
day for several years, are they not?
Yes, usually somewhere between 3-15 DWPD (Drive Writes Per Day) - it
typically works out at around 5000 full drive write cycles for
enterprise drives. However, at both the low capacity end of the
scale or the high performance end (i.e. pcie cards capable of multiple
GB/s writes), it's not uncommon to be able to burn a DW cycle in
under 10 minutes and so you can easily burn the life out of a drive
in a couple of weeks of intense testing....
So such a test can take weeks
or months, depending on the ratio between disk size and bandwidth.
Still, I guess it has to be done.
*nod*
Cheers,
Dave.
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