[linux-lvm] performance comparison soft-hardware RAID + LVM: bad
Ron Arts
raarts at netland.nl
Wed Oct 16 11:04:10 UTC 2002
Jon Bendtsen wrote:
> Ron Arts wrote:
>
[snip]
(I received your report, thanks)
>
>
>>Results are below. Anyone care to comment? Especially LVM performance
>>disappointed here.
>
>
> I cant clearly see what is LVM setup and what isnt. Remember that LVM
> doesnt allocate blocks sequeltial, but by default the first one free.
> So, when you create 3 lv's, and then you mkfs them, then you allocate
> at least the first block. Then when you fill the rest of the
> filesystem...
> you allocate the next blocks. Results are one block in the beginning,
> a wide gap, and then the rest of the blocks.
>
Sorry, I don't understand. Why the gap?
Omn the other hand, the underlying devices are RAID-1 in software, the
allocation shouldn't matter should it?
>
>
>>LVM machine setup:
>>
>>2 18Gb disks. I created 3 partitions on both disks, 128Mb, 512Mb and 17Gb
>>Equal partitions were combined into RAID-1 devices (md driver).
>>First md device mounted on /boot, second for swapfile, and third
>>as basis for LVM
>>
>>Out of the volume group four LV were created and mounted as follows:
>>
>>[root at nbs-126 root]# df
>>Filesystem 1K-blocks Used Available Use% Mounted on
>>/dev/vg0/root 4225092 1293064 2717400 33% /
>>/dev/md0 124323 11517 106387 10% /boot
>>/dev/vg0/home 4225092 32828 3977636 1% /home
>>none 514996 0 514996 0% /dev/shm
>>/dev/vg0/var 4225092 51720 3958744 2% /var
>>/dev/vg0/mysql 16513960 32828 15642272 1% /var/lib/mysql
>>
>>Is there a reason for the performance degradation I saw with LVM?
>
>
> I've done 3 (or 0.5 + 0.5 + 1) benchmarks. The first 2 times i didnt do
> it well enough. I dont believe you have done it well enough, you clearly
> dont have enough numbers. I found that using tiobench i had to variate
> the number of threads (concurrent read/write) and the blocksize, before
> i
> got the best performance. And it variates alot. (See my .pdf, which i
> will
> mail to you). I've got lots of numbers. I used gnuplot to create graphs,
Okay, but lots of numbers still don't explain why in this particular case
performance was so slow. If I understand why, I can begin to make
optimizations.
To give some background:
I do this because I need such a setup for a particular application
(MySQL high volume logging server). If I understand the issues involved
I can make more informed choices implementing the application.
Should it log using multiple threads or one? Will readers from the
datbase hinder the writing process a lot? What is the best way
to add disks using LVM, without taking a large performance hit?
This server must be up 24x7. I found something called scsirastools
that can deal with hotswapping SCSI disks under software RAID.
I thought I'd first try some benchmarks with bonnie to get a feel
for the issues involved, and seeing the performance (and CPU) hit
for my LVM setup (and having never used LVM before) I decided
to ask you guys about this.
And thanks for your report, at least it confirmed what I
had seen: software raid is faster then hardware.
Regards,
Ron Arts
--
Netland Internet Services
bedrijfsmatige internetoplossingen
http://www.netland.nl Kruislaan 419 1098 VA Amsterdam
info: 020-5628282 servicedesk: 020-5628280 fax: 020-5628281
Does old mail ever arrive?
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 3291 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://listman.redhat.com/archives/linux-lvm/attachments/20021016/0c833cd3/attachment.bin>
More information about the linux-lvm
mailing list