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

[Håkon Alstadheim <hakon@xxxxxxxxxxxxxxxxxx>]

Den 16.12.2018 17:42, skrev frm@xxxxxxxxxxx:
> My disk performance of Xen guest is very slow. My latest configuration:
>
> At guest, I have:
>
> `    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, 15.6421 s, 68.6 MB/s
> `
>
> At Dom0 I have:
>
> `    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, 3.81855 s, 281 MB/s
> `
>
> The dom0 is about 5 times faster than the guest.
Worse here, 15 to 20 x slower (I did a few runs just now, numbers vary 
by quite a lot, but the magnitude difference is consistent). Same md 
partition mounted on dom0 and as domu root. Md constituents are SAS 
spinning rust. In the past I've tried looking at tunables for the domu 
disks, to no effect. I would love a "domu disk tuning for dummies" . 
Throughput speed is tolerable usually, but when a rebuild is running on 
the disk-array, the domus are all but unusable interactively. I've been 
poking at this issue for a couple of years, not getting anywhere. The 
poking has been totally devoid of proper notes and a strategy, but over 
time I have ditched LVM and tried various schedulers, triple-checked 
alignment, used both xfs and ext4, all without ever seeing a definitive 
break-through. I had bcache with an m2 ssd on top of LVM for a while 
until my m2 wore out. That helped paper over the issue. (bcache on top 
of virtual devices is not recommended, btw. If done, make sure you use 
no-discard on the disk-configuration for the vm)
Xentop:

xentop - 20:06:04   Xen 4.11.1
     NAME  STATE   CPU(sec) CPU(%)     MEM(k) MEM(%) VCPUS
  Domain-0 -----r      33821    8.1    4995856    7.5 8
 steam.hvm --b---         51    0.3    7332136   11.0 6

Guest steam:

# df -hT  /tmp
Filsystem      Type Størrelse Brukt Tilgj. Bruk% Montert på
/dev/xvdb      ext4      196G  162G    25G   87% /
# cd /tmp
# 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, 60,1407 s, 17,9 MB/s

Dom0:

 # df -hT ./
Filesystem     Type  Size  Used Avail Use% Mounted on

/dev/md2p8     ext4  196G  163G   24G  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, 5.37238 s, 200 MB/s



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