

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

Jon Bendtsen wrote:
Ron Arts wrote:


(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
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
got the best performance. And it variates alot. (See my .pdf, which i
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.

Ron Arts

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