Re: [Xen-users] Disk performance on guest incredibly low.

[Håkon Alstadheim <hakon@xxxxxxxxxxxxxxxxxx>]

Den 17.12.2018 10:32, skrev Roalt Zijlstra | webpower:
> Hey Håkon,
>
> One little tip on disk IO in Virtual hosts, is changing the FS 
> scheduler from cfq to noop. In our Xen PV configs we add this to the 
> xen.cfg files at the end:
> extra="clocksource=tsc elevator=noop"
>
> Especially the "elevator=noop" parameter is forcing all FS-es to the 
> noop scheduler. In my experience that gave our Xenservers a pretty 
> nice boost.
> Red Hat does recoomend this for all Virtual servers (see the link 
> below). To test this you don't need to reboot at all.
> For example from the link below there is a code snippet about getting 
> and setting the scheduler for /dev/hda. (Replace hda with sdb or any 
> other block device)
> # cat /sys/block/hda/queue/scheduler
> noop anticipatory deadline [cfq]
>
> # echo 'noop' > /sys/block/hda/queue/scheduler
> # cat /sys/block/hda/queue/scheduler
> [noop] anticipatory deadline cfq
>
> More info: https://access.redhat.com/solutions/5427
Yes, found that resource some time ago. Tried again now, no 
break-through. I've got 'none' available as scheduler, rather than noop, 
but they should be equal. No matter what I do (in dom0 and/or domu) I 
get at least 10 x higher speed in the dom0. :-/ .
Example:

###

###In dom0, md-raid on drives f j g h i k. echo-and-cat does the obvious.

### (I usually run with mq-deadline as scheduler. seems to give 
marginally better performance)
# for f in f j g h i k ; do echo-and-cat 
/sys/block/sd${f}/queue/scheduler;done
/sys/block/sdf/queue/scheduler
[none] mq-deadline kyber bfq
/sys/block/sdj/queue/scheduler
[none] mq-deadline kyber bfq
/sys/block/sdg/queue/scheduler
[none] mq-deadline kyber bfq
/sys/block/sdh/queue/scheduler
[none] mq-deadline kyber bfq
/sys/block/sdi/queue/scheduler
[none] mq-deadline kyber bfq
/sys/block/sdk/queue/scheduler
[none] mq-deadline kyber bfq
# mount /dev/disk/by-label/SAS-STEAM /mnt/tull
# cd /mnt/tull/tmp
# df -hT ./
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/md2p8     ext4  196G  164G   23G  88% /mnt/tull
# dd if=/dev/zero of=a_file bs=1M count=1024 conv=fsync oflag=direct
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 4.56845 s, 235 MB/s
0:root@gentoo tmp # dd if=/dev/zero of=a_file bs=1M count=1024 
conv=fsync oflag=direct
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 7.52557 s, 143 MB/s
###

### Booting domu with root and /tmp on SAS-STEAM, ssh into domu

###

# cd /tmp
# df -hT ./
Filsystem      Type Størrelse Brukt Tilgj. Bruk% Montert på
/dev/xvdb      ext4      196G  163G    24G   88% /
# cat /sys/block/xvdb/
alignment_offset   device/            holders/ power/             
ro                 subsystem/
bdi/               discard_alignment  inflight queue/             
size               trace/
capability         ext_range          integrity/ range              
slaves/            uevent
dev                hidden             mq/ removable          stat
# cat /sys/block/xvdb/queue/scheduler
[none] mq-deadline
# cd /tmp
# df -hT ./
Filsystem      Type Størrelse Brukt Tilgj. Bruk% Montert på
/dev/xvdb      ext4      196G  163G    24G   88% /
# cat /sys/block/xvdb/
alignment_offset   device/            holders/ power/             
ro                 subsystem/
bdi/               discard_alignment  inflight queue/             
size               trace/
capability         ext_range          integrity/ range              
slaves/            uevent
dev                hidden             mq/ removable          stat
# cat /sys/block/xvdb/queue/scheduler
[none] mq-deadline
# dd if=/dev/zero of=a_file bs=1M count=1024 conv=fsync oflag=direct
1024+0 oppføringer inn
1024+0 oppføringer ut
1073741824 byte (1,1 GB, 1,0 GiB) kopiert, 64,2402 s, 16,7 MB/s
# dd if=/dev/zero of=a_file bs=1M count=1024 conv=fsync oflag=direct
1024+0 oppføringer inn
1024+0 oppføringer ut
1073741824 byte (1,1 GB, 1,0 GiB) kopiert, 59,7517 s, 18,0 MB/s
#





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