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Slow disk performance on Xen guest


Mark Alliban

Question

Hello,

I am using XenServer 7.4 on a Dell server with 6x 1TB HD, in a Raid 5 array.

Guest is Windows Server 2016 with the Xen Tools installed.

I'm getting good performance on the XenServer itself:

 

[root@XenServer ~]# hdparm -Tt /dev/sda

/dev/sda:
 Timing cached reads:   10014 MB in  1.99 seconds = 5029.19 MB/sec
 Timing buffered disk reads: 234 MB in  3.17 seconds =  73.83 MB/sec

[root@XenServer ~]# dd if=/dev/zero of=/tmp/output bs=8k count=10k; rm -f /tmp/output
10240+0 records in
10240+0 records out
83886080 bytes (84 MB) copied, 1.44844 s, 57.9 MB/s

 

But on the guest it's really slow:

Image1.thumb.jpg.5b0dfb4604a3e9892b04ff69e415e33e.jpg

 

There's no other guests and the Xen Tools are installed.

Any ideas??

 

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Thanks for the suggestions, here's bonnie++ output:

 

[root@XenServer bonnie++-1.03e]# bonnie++ -d /tmp -r 250M -s 500M -b -u root
Using uid:0, gid:0.
Writing with putc()...done
Writing intelligently...done
Rewriting...done
Reading with getc()...done
Reading intelligently...done
start 'em...done...done...done...
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version 1.03e       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
XenServer      500M 21057  30 21583   5 13337   3 42350  54 97501  10 458.3   1
                    ------Sequential Create------ --------Random Create--------
                    -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16    50   0 +++++ +++    50   0    51   0 +++++ +++    36   0

 

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I'd use a size around 10x that of your available memory in bonnie++ to be sure the cache plays an insignificant role. That said, what you show above isn't all that great, alas.

How is your RAID set up, in particular, what are your disks (SAS, SATA, SSD, etc.), and their speeds? To get really good reads, you often need a RAID10 or better configuration, we've found. Even with a pure SSD drive-based RAID 5 configuration, our writes are not good with one array containing four SSD drives and one spare.

 

-=Tobias

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Here's a larger test, seems similar.

There's 6x SASu drives, 4 of them in a RAID5 array with 2 hot spares.

I'm not looking for stunning performance, but with what I've got I think I should be getting a lot better than 8MB/s on a Windows guest. It's certainly a lot slower than I'm getting on other servers.

[root@XenServer ~]# bonnie++ -d /tmp -r 100M -s 2000M -b -u root
Using uid:0, gid:0.
Writing with putc()...done
Writing intelligently...done
Rewriting...done
Reading with getc()...done
Reading intelligently...done
start 'em...done...done...done...
Create files in sequential order...done.
Stat files in sequential order...done.
Delete files in sequential order...done.
Create files in random order...done.
Stat files in random order...done.
Delete files in random order...done.
Version 1.03e       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
XenServer     2000M 20670  29 20327   5 13811   3 62012  79 184192  19 239.7   1
                    ------Sequential Create------ --------Random Create--------
                    -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16    55   0 +++++ +++    56   0    41   0 +++++ +++    23   0


 

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A four-disk RAID5 isn't going to do great especially with writes. For good writes, you really need RAID10 or better.

I have a 4-disk RAID5 comprised of just four fast SSD drives and it oesn't even do that great. The saving grace is a SDS (software-defined storage) array with a good read cache.

 

-=Tobias

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