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bbfa235
(Get the code!)
BugLink:
[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.
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.
After digging in git log between 4.4 and 4.6, this commit stood out:
commit 9c573de3283af00
Author: Shaohua Li <email address hidden>
Date: Mon Apr 25 16:52:38 2016 -0700
Subject: MD: make bio mergeable
You can read it here:
https:/
"blk_queue_split marks bio unmergeable, which makes sense for normal bio.
But if dispatching the bio to underlayer disk, the blk_queue_split
checks are invalid, hence it's possible the bio becomes mergeable."
The snapshot is dm-3, and it has two underlayer disks, dm-1 and nvme1n1. Which
means we qualify for merging.
Looking at the xenial 4.4 kernel tree, this commit is actually already applied,
since it got backported to the mainline 4.4 kernel:
https://<email address hidden>
So why is xenial effected? Looking at the bugzilla page for that commit:
https:/
We see that merging is controlled by a sysfs entry,
/sys/block/
On 4.4 xenial, reading from this yields 2, or (QUEUE_
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/
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_
commit ef2d4615c59efb3
Author: Keith Busch <email address hidden>
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/
Please cherry-pick ef2d4615c59efb3
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/
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/
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.
real 0m42.693s
user 0m0.008s
sys 0m0.388s
$ cat /sys/block/
2
On a patched xenial kernel, performance is restored:
4.4.0-151-generic #178+hf228435v2
$ time cat /tmp/mount.
real 0m1.773s
user 0m0.008s
sys 0m0.184s
$ cat /sys/block/
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:/
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: 488991e28e55b4f
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
ef2d4615c59efb3
linux-image-
linux-image-aws: https:/