On 5/7/19 5:40 AM, Lukas Czerner wrote:
On Tue, May 07, 2019 at 10:48:55AM +0200, Jan Tulak wrote:
On Tue, May 7, 2019 at 9:10 AM Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
On Mon, May 06, 2019 at 04:56:44PM -0400, Ric Wheeler wrote:
...
* Whole device discard at the block level both for a device that has been
completely written and for one that had already been trimmed
Yes, usefull. Also note that a long time ago when I've done the testing
I noticed that after a discard request, especially after whole device
discard, the read/write IO performance went down significanly for some
drives. I am sure things have changed, but I think it would be
interesting to see how does it behave now.
My understanding of how drives (not just SSD's but they are the main
target here) can handle a discard can vary a lot, including:
* just ignore it for any reason and not return a failure - it is just a
hint by spec.
* update metadata to mark that region as unused and then defer any real
work to later (like doing wear level stuff, pre-erase for writes, etc).
This can have a post-discard impact. I think of this kind of like
updating page table entries for virtual memory - low cost update now,
all real work deferred.
* do everything as part of the command - this can be relatively slow,
most of the cost of a write I would guess (i.e., go in and over-write
the region with zeros or just do the erase of the flash block under the
region).
Your earlier work supports the need to test IO performance after doing
the trims/discards - we might want to test it right away, then see if
waiting 10 minutes, 30 minutes, etc helps?
* Discard performance at the block level for 4k discards for a device that
has been completely written and again the same test for a device that has
been completely discarded.
* Same test for large discards - say at a megabyte and/or gigabyte size?
From my testing (again it was long time ago and things probably changed
since then) most of the drives I've seen had largely the same or similar
timing for discard request regardless of the size (hence, the conclusion
was the bigger the request the better). A small variation I did see
could have been explained by kernel implementation and discard_max_bytes
limitations as well.
* Same test done at the device optimal discard chunk size and alignment
Should the discard pattern be done with a random pattern? Or just
sequential?
I think that all of the above will be interesting. However there are two
sides of it. One is just pure discard performance to figure out what
could be the expectations and the other will be "real" workload
performance. Since from my experience discard can have an impact on
drive IO performance beyond of what's obvious, testing mixed workload
(IO + discard) is going to be very important as well. And that's where
fio workloads can come in (I actually do not know if fio already
supports this or not).
Really good points. I think it is probably best to test just at the
block device level to eliminate any possible file system interactions
here. The lessons learned though might help file systems handle things
more effectively?
And:
On Tue, May 7, 2019 at 10:22 AM Nikolay Borisov <nborisov@xxxxxxxx> wrote:
I have some vague recollection this was brought up before but how sure
are we that when a discard request is sent down to disk and a response
is returned the actual data has indeed been discarded. What about NCQ
effects i.e "instant completion" while doing work in the background. Or
ignoring the discard request altogether?
As Nikolay writes in the other thread, I too have a feeling that there
have been a discard-related discussion at LSF/MM before. And if I
remember, there were hints that the drives (sometimes) do asynchronous
trim after returning a success. Which would explain the similar time
for all sizes and IO drop after trim.
Yes, that was definitely the case in the past. It's also why we've
seen IO performance drop after a big (whole device) discard as the
device was busy in the background.
For SATA specifically, there was a time when the ATA discard command was
not queued so we had to drain all other pending requests, do the
discard, and then resume. This was painfully slow then (not clear that
this was related to the performance impact you saw - it would be an
impact I think for the next few dozen commands?).
The T13 people (and most drives I hope) fixed this years back to be a
queued command so we don't have that same concern now I think.
However Nikolay does have a point. IIRC device is free to ignore discard
requests, I do not think there is any reliable way to actually tell that
the data was really discarded. I can even imagine a situation that the
device is not going to do anything unless it's pass some threshold of
free blocks for wear leveling. If that's the case our tests are not
going to be very useful unless they do stress such corner cases. But
that's just my speculation, so someone with a better knowledge of what
vendors are doing might tell us if it's something to worry about or not.
The way I think of it is our "nirvana" state for discard would be:
* all drives have very low cost discard commands with minimal
post-discard performance impact on the normal workload which would let
us issue the in-band discards (-o discard mount option)
* drives might still (and should be expected to) ignore some of these
commands so freed and "discarded" space might still not be really discarded.
* we will still need to run a periodic (once a day? a week?) fstrim to
give the drive a chance to clean up anything even when using "mount -o
discard". Of course, the fstrim size is bigger I expect than the size
from inband discard so testing larger sizes will be important.
Does this make sense?
Ric
So, I think that the mixed workload (IO + discard) is a pretty
important part of the whole topic and a pure discard test doesn't
really tell us anything, at least for some drives.
I think both are important especially since mixed IO tests are going to
be highly workload specific.
-Lukas
Jan
--
Jan Tulak
