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Message #30784
[Bug 1833319] [NEW] Performance degradation when copying from LVM snapshot backed by NVMe disk
Public bug reported:
BugLink: https://bugs.launchpad.net/bugs/1833319
[Impact]
When copying files from a mounted LVM snapshot which resides on NVMe storage devices, there is a massive performance degradation in the rate sectors are read from the disk.
The kernel is not merging sector requests and is instead issuing many small
sector requests to the NVMe storage controller instead of one larger request.
Experiments have shown a 14x-25x performance degradation in reads, where
copies used to take seconds, now take minutes, and copies which took
thirty minutes now take many hours.
[Fix]
The following was found with btrace, running alongside cat (see
Testing):
Standard lvm copy:
$ cat /mnt/dummy1 1> /dev/null
LVM snapshot copy:
$ cat /tmp/mount.backup_OXV/dummy2 1> /dev/null
Tracing:
# btrace /dev/nvme1n1 > trace.data
Looking at the "control" case, of copying from /mnt, which is the
standard lvm volume, see see a trace like:
259,0 1 13 0.002545516 1579 A R 280576 + 512 <- (252,0) 278528
259,0 1 14 0.002545701 1579 Q R 280576 + 512 [cat]
259,0 1 15 0.002547020 1579 G R 280576 + 512 [cat]
259,0 1 16 0.002547631 1579 U N [cat] 1
259,0 1 17 0.002547775 1579 I RS 280576 + 512 [cat]
259,0 1 18 0.002551381 1579 D RS 280576 + 512 [cat]
259,0 1 19 0.004099666 0 C RS 280576 + 512 [0]
A = IO remapped to different device
Q = IO handled by request queue
G = Get request
U = Unplug request
I = IO inserted onto request queue
D = IO issued to driver
C = IO completion
Firstly, the request is mapped from a different device, from /mnt which
is dm-1 to the nvme disk. A 512 sector read is placed on the IO request
queue, where it is then inserted into the driver request queue and then
the driver is commanded to fetch the data, and then it completes.
Now, when reading from the LVM snapshot, we see:
259,0 1 113 0.001117160 1606 A R 837872 + 8 <- (252,0) 835824
259,0 1 114 0.001117276 1606 Q R 837872 + 8 [cat]
259,0 1 115 0.001117451 1606 G R 837872 + 8 [cat]
259,0 1 116 0.001117979 1606 A R 837880 + 8 <- (252,0) 835832
259,0 1 117 0.001118119 1606 Q R 837880 + 8 [cat]
259,0 1 118 0.001118285 1606 G R 837880 + 8 [cat]
259,0 1 122 0.001121613 1606 I RS 837640 + 8 [cat]
259,0 1 123 0.001121687 1606 I RS 837648 + 8 [cat]
259,0 1 124 0.001121758 1606 I RS 837656 + 8 [cat]
...
259,0 1 154 0.001126118 377 D RS 837648 + 8 [kworker/1:1H]
259,0 1 155 0.001126445 377 D RS 837656 + 8 [kworker/1:1H]
259,0 1 156 0.001126871 377 D RS 837664 + 8 [kworker/1:1H]
...
259,0 1 183 0.001848512 0 C RS 837632 + 8 [0]
Now what is happening here, is that a request for 8 sector read is
placed onto the IO request queue, and is then inserted one at a time to
the driver request queue and then fetched by the driver.
Comparing this behaviour to reading data from a LVM snapshot on 4.6 mainline+
or the Ubuntu 4.15 HWE kernel:
M = IO back merged with request on queue
259,0 0 194 0.000532515 1897 A R 7358960 + 8 <- (253,0) 7356912
259,0 0 195 0.000532634 1897 Q R 7358960 + 8 [cat]
259,0 0 196 0.000532810 1897 M R 7358960 + 8 [cat]
259,0 0 197 0.000533864 1897 A R 7358968 + 8 <- (253,0) 7356920
259,0 0 198 0.000533991 1897 Q R 7358968 + 8 [cat]
259,0 0 199 0.000534177 1897 M R 7358968 + 8 [cat]
259,0 0 200 0.000534474 1897 UT N [cat] 1
259,0 0 201 0.000534586 1897 I R 7358464 + 512 [cat]
259,0 0 202 0.000537055 1897 D R 7358464 + 512 [cat]
259,0 0 203 0.002242539 0 C R 7358464 + 512 [0]
This shows us a 8 sector read is added to the request queue, and is then
subsequently [M]erged backward with other requests on the queue until the sum of all of those merged requests becomes 512 sectors. From there, the 512 sector read is placed onto the IO queue, where it is fetched by the device driver, and completes.
The problem is that the 4.4 xenial kernel is not merging 8 sector
requests.
I came across this bugzilla entry,
https://bugzilla.kernel.org/show_bug.cgi?id=117051
and we see that merging is controlled by a sysfs entry,
/sys/block/nvme1n1/queue/nomerges
On 4.4 xenial, reading from this yields 2, or (QUEUE_FLAG_NOMERGES).
On 4.6+ and 4.15 HWE kernel, reading from this yields 0, or allowing merge.
Setting this to 0 on the 4.4 kernel with:
# echo "0" > /sys/block/nvme1n1/queue/nomerges
and testing again, we find performance is restored and the problem is
fixed.
Looking at the trace with btrace, we see that performs 8 sector reads,
which get backmerged into a 512 sector read which is done in one go.
Looking into the kernel tree with cscope, on QUEUE_FLAG_NOMERGES, we
come across
commit ef2d4615c59efb312e531a5e949970f37ca1c841
Author: Keith Busch <keith.busch@xxxxxxxxx>
Date: Thu Feb 11 13:05:40 2016 -0700
Subject: NVMe: Allow request merges
This commit removes the QUEUE_FLAG_NOMERGES flag from being set during
driver init, allowing requests to be backmerged. This also has a direct
effect of defaulting /sys/block/nvme1n1/queue/nomerges to 0.
Please cherry-pick ef2d4615c59efb312e531a5e949970f37ca1c841 to all xenial 4.4
kernels.
[Testcase]
You can replicate the problem with a system with a NVMe disk. I
recommend using c5.large AWS EC2 instances with a secondary gpt2 EBS
disk of 200gb or larger.
Steps (with NVMe disk being /dev/nvme1n1):
1. sudo pvcreate /dev/nvme1n1
2. sudo vgcreate secvol /dev/nvme1n1
3. sudo lvcreate --name seclv -l 80%FREE secvol
4. sudo mkfs.ext4 /dev/secvol/seclv
5. sudo mount /dev/mapper/secvol-seclv /mnt
6. for i in `seq 1 20`; do sudo dd if=/dev/zero of=/mnt/dummy$i bs=512M count=1; done
7. sudo lvcreate --snapshot /dev/secvol/seclv --name tmp_backup1 --extents '90%FREE'
8. NEWMOUNT=$(mktemp -t -d mount.backup_XXX)
9. sudo mount -v -o ro /dev/secvol/tmp_backup1 $NEWMOUNT
To replicate, simply read one of those 512mb files:
10. time cat $NEWMOUNT/dummy1 1> /dev/null
On a stock xenial kernel, expect to see the following:
4.4.0-151-generic #178-Ubuntu
$ time cat /tmp/mount.backup_TYD/dummy1 1> /dev/null
real 0m42.693s
user 0m0.008s
sys 0m0.388s
$ cat /sys/block/nvme1n1/queue/nomerges
2
On a patched xenial kernel, performance is restored:
4.4.0-151-generic #178+hf228435v20190618b1-Ubuntu
$ time cat /tmp/mount.backup_aId/dummy1 1> /dev/null
real 0m1.773s
user 0m0.008s
sys 0m0.184s
$ cat /sys/block/nvme1n1/queue/nomerges
0
[Regression Potential]
Cherry picking "NVMe: Allow request merges" changes the default request
policy for NVMe drives, which may give some cause for concern in both
terms of stability and performance for other workloads.
Regarding stability, this flag was originally set when the NVMe driver was
bio based, before the driver had been converted to blk-mq and separated out from /block. You can read a mailing list thread about it here:
https://lists.infradead.org/pipermail/linux-
nvme/2016-February/003946.html
Along with the commit "MD: make bio mergeable" there is no reason to not
allow requests to be mergeable for the new NVMe driver.
Regarding performance for other workloads, I reference the commit which
QUEUE_FLAG_NOMERGES or nomerges == 2 was introduced:
commit: 488991e28e55b4fbca8067edf0259f69d1a6f92c
subject: block: Added in stricter no merge semantics for block I/O
nomerges Throughput %System Improvement (tput / %sys)
-------- ------------ ----------- -------------------------
0 12.45 MB/sec 0.669365609
1 12.50 MB/sec 0.641519199 0.40% / 2.71%
2 12.52 MB/sec 0.639849750 0.56% / 2.96%
It shows a 0.56% performance increase for no merging / 2, over allowing
merging / 0 for random IO workloads.
Comparing this with the 14x-25x performance degradation for reads where
requests are not able to be merged, it is clear that changing the
default to 0 will not impact any other workloads by any significant
margin.
The commit is also present in Linux 4.5 mainline, can be cleanly cherry
picked and is still present in the kernel to this day, and after review
of the NVMe driver, I believe there will be no regressions.
If you are interested in testing, I have prepared two ppas with
ef2d4615c59efb312e531a5e949970f37ca1c841 patched:
linux-image-generic: https://launchpad.net/~mruffell/+archive/ubuntu/sf228435-test-generic
linux-image-aws: https://launchpad.net/~mruffell/+archive/ubuntu/sf228435-test
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Affects: linux (Ubuntu Xenial)
Importance: Undecided
Assignee: Matthew Ruffell (mruffell)
Status: In Progress
** Tags: sts
** Also affects: linux (Ubuntu Xenial)
Importance: Undecided
Status: New
** Tags added: sts
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Matching subscriptions: Ubuntu 16.04 Bugs
https://bugs.launchpad.net/bugs/1833319
Title:
Performance degradation when copying from LVM snapshot backed by NVMe
disk
Status in linux package in Ubuntu:
New
Status in linux source package in Xenial:
In Progress
Bug description:
BugLink: https://bugs.launchpad.net/bugs/1833319
[Impact]
When copying files from a mounted LVM snapshot which resides on NVMe storage devices, there is a massive performance degradation in the rate sectors are read from the disk.
The kernel is not merging sector requests and is instead issuing many small
sector requests to the NVMe storage controller instead of one larger request.
Experiments have shown a 14x-25x performance degradation in reads,
where copies used to take seconds, now take minutes, and copies which
took thirty minutes now take many hours.
[Fix]
The following was found with btrace, running alongside cat (see
Testing):
Standard lvm copy:
$ cat /mnt/dummy1 1> /dev/null
LVM snapshot copy:
$ cat /tmp/mount.backup_OXV/dummy2 1> /dev/null
Tracing:
# btrace /dev/nvme1n1 > trace.data
Looking at the "control" case, of copying from /mnt, which is the
standard lvm volume, see see a trace like:
259,0 1 13 0.002545516 1579 A R 280576 + 512 <- (252,0) 278528
259,0 1 14 0.002545701 1579 Q R 280576 + 512 [cat]
259,0 1 15 0.002547020 1579 G R 280576 + 512 [cat]
259,0 1 16 0.002547631 1579 U N [cat] 1
259,0 1 17 0.002547775 1579 I RS 280576 + 512 [cat]
259,0 1 18 0.002551381 1579 D RS 280576 + 512 [cat]
259,0 1 19 0.004099666 0 C RS 280576 + 512 [0]
A = IO remapped to different device
Q = IO handled by request queue
G = Get request
U = Unplug request
I = IO inserted onto request queue
D = IO issued to driver
C = IO completion
Firstly, the request is mapped from a different device, from /mnt
which is dm-1 to the nvme disk. A 512 sector read is placed on the IO
request queue, where it is then inserted into the driver request queue
and then the driver is commanded to fetch the data, and then it
completes.
Now, when reading from the LVM snapshot, we see:
259,0 1 113 0.001117160 1606 A R 837872 + 8 <- (252,0) 835824
259,0 1 114 0.001117276 1606 Q R 837872 + 8 [cat]
259,0 1 115 0.001117451 1606 G R 837872 + 8 [cat]
259,0 1 116 0.001117979 1606 A R 837880 + 8 <- (252,0) 835832
259,0 1 117 0.001118119 1606 Q R 837880 + 8 [cat]
259,0 1 118 0.001118285 1606 G R 837880 + 8 [cat]
259,0 1 122 0.001121613 1606 I RS 837640 + 8 [cat]
259,0 1 123 0.001121687 1606 I RS 837648 + 8 [cat]
259,0 1 124 0.001121758 1606 I RS 837656 + 8 [cat]
...
259,0 1 154 0.001126118 377 D RS 837648 + 8 [kworker/1:1H]
259,0 1 155 0.001126445 377 D RS 837656 + 8 [kworker/1:1H]
259,0 1 156 0.001126871 377 D RS 837664 + 8 [kworker/1:1H]
...
259,0 1 183 0.001848512 0 C RS 837632 + 8 [0]
Now what is happening here, is that a request for 8 sector read is
placed onto the IO request queue, and is then inserted one at a time
to the driver request queue and then fetched by the driver.
Comparing this behaviour to reading data from a LVM snapshot on 4.6 mainline+
or the Ubuntu 4.15 HWE kernel:
M = IO back merged with request on queue
259,0 0 194 0.000532515 1897 A R 7358960 + 8 <- (253,0) 7356912
259,0 0 195 0.000532634 1897 Q R 7358960 + 8 [cat]
259,0 0 196 0.000532810 1897 M R 7358960 + 8 [cat]
259,0 0 197 0.000533864 1897 A R 7358968 + 8 <- (253,0) 7356920
259,0 0 198 0.000533991 1897 Q R 7358968 + 8 [cat]
259,0 0 199 0.000534177 1897 M R 7358968 + 8 [cat]
259,0 0 200 0.000534474 1897 UT N [cat] 1
259,0 0 201 0.000534586 1897 I R 7358464 + 512 [cat]
259,0 0 202 0.000537055 1897 D R 7358464 + 512 [cat]
259,0 0 203 0.002242539 0 C R 7358464 + 512 [0]
This shows us a 8 sector read is added to the request queue, and is then
subsequently [M]erged backward with other requests on the queue until the sum of all of those merged requests becomes 512 sectors. From there, the 512 sector read is placed onto the IO queue, where it is fetched by the device driver, and completes.
The problem is that the 4.4 xenial kernel is not merging 8 sector
requests.
I came across this bugzilla entry,
https://bugzilla.kernel.org/show_bug.cgi?id=117051
and we see that merging is controlled by a sysfs entry,
/sys/block/nvme1n1/queue/nomerges
On 4.4 xenial, reading from this yields 2, or (QUEUE_FLAG_NOMERGES).
On 4.6+ and 4.15 HWE kernel, reading from this yields 0, or allowing merge.
Setting this to 0 on the 4.4 kernel with:
# echo "0" > /sys/block/nvme1n1/queue/nomerges
and testing again, we find performance is restored and the problem is
fixed.
Looking at the trace with btrace, we see that performs 8 sector reads,
which get backmerged into a 512 sector read which is done in one go.
Looking into the kernel tree with cscope, on QUEUE_FLAG_NOMERGES, we
come across
commit ef2d4615c59efb312e531a5e949970f37ca1c841
Author: Keith Busch <keith.busch@xxxxxxxxx>
Date: Thu Feb 11 13:05:40 2016 -0700
Subject: NVMe: Allow request merges
This commit removes the QUEUE_FLAG_NOMERGES flag from being set during
driver init, allowing requests to be backmerged. This also has a
direct effect of defaulting /sys/block/nvme1n1/queue/nomerges to 0.
Please cherry-pick ef2d4615c59efb312e531a5e949970f37ca1c841 to all xenial 4.4
kernels.
[Testcase]
You can replicate the problem with a system with a NVMe disk. I
recommend using c5.large AWS EC2 instances with a secondary gpt2 EBS
disk of 200gb or larger.
Steps (with NVMe disk being /dev/nvme1n1):
1. sudo pvcreate /dev/nvme1n1
2. sudo vgcreate secvol /dev/nvme1n1
3. sudo lvcreate --name seclv -l 80%FREE secvol
4. sudo mkfs.ext4 /dev/secvol/seclv
5. sudo mount /dev/mapper/secvol-seclv /mnt
6. for i in `seq 1 20`; do sudo dd if=/dev/zero of=/mnt/dummy$i bs=512M count=1; done
7. sudo lvcreate --snapshot /dev/secvol/seclv --name tmp_backup1 --extents '90%FREE'
8. NEWMOUNT=$(mktemp -t -d mount.backup_XXX)
9. sudo mount -v -o ro /dev/secvol/tmp_backup1 $NEWMOUNT
To replicate, simply read one of those 512mb files:
10. time cat $NEWMOUNT/dummy1 1> /dev/null
On a stock xenial kernel, expect to see the following:
4.4.0-151-generic #178-Ubuntu
$ time cat /tmp/mount.backup_TYD/dummy1 1> /dev/null
real 0m42.693s
user 0m0.008s
sys 0m0.388s
$ cat /sys/block/nvme1n1/queue/nomerges
2
On a patched xenial kernel, performance is restored:
4.4.0-151-generic #178+hf228435v20190618b1-Ubuntu
$ time cat /tmp/mount.backup_aId/dummy1 1> /dev/null
real 0m1.773s
user 0m0.008s
sys 0m0.184s
$ cat /sys/block/nvme1n1/queue/nomerges
0
[Regression Potential]
Cherry picking "NVMe: Allow request merges" changes the default
request policy for NVMe drives, which may give some cause for concern
in both terms of stability and performance for other workloads.
Regarding stability, this flag was originally set when the NVMe driver was
bio based, before the driver had been converted to blk-mq and separated out from /block. You can read a mailing list thread about it here:
https://lists.infradead.org/pipermail/linux-
nvme/2016-February/003946.html
Along with the commit "MD: make bio mergeable" there is no reason to
not allow requests to be mergeable for the new NVMe driver.
Regarding performance for other workloads, I reference the commit which
QUEUE_FLAG_NOMERGES or nomerges == 2 was introduced:
commit: 488991e28e55b4fbca8067edf0259f69d1a6f92c
subject: block: Added in stricter no merge semantics for block I/O
nomerges Throughput %System Improvement (tput / %sys)
-------- ------------ ----------- -------------------------
0 12.45 MB/sec 0.669365609
1 12.50 MB/sec 0.641519199 0.40% / 2.71%
2 12.52 MB/sec 0.639849750 0.56% / 2.96%
It shows a 0.56% performance increase for no merging / 2, over allowing
merging / 0 for random IO workloads.
Comparing this with the 14x-25x performance degradation for reads
where requests are not able to be merged, it is clear that changing
the default to 0 will not impact any other workloads by any
significant margin.
The commit is also present in Linux 4.5 mainline, can be cleanly
cherry picked and is still present in the kernel to this day, and
after review of the NVMe driver, I believe there will be no
regressions.
If you are interested in testing, I have prepared two ppas with
ef2d4615c59efb312e531a5e949970f37ca1c841 patched:
linux-image-generic: https://launchpad.net/~mruffell/+archive/ubuntu/sf228435-test-generic
linux-image-aws: https://launchpad.net/~mruffell/+archive/ubuntu/sf228435-test
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1833319/+subscriptions
Follow ups
