Re: [linux-lvm] performance comparison soft-hardware RAID + LVM: bad

[Ron Arts <raarts@netland.nl>]

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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@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


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