Ron Arts wrote:
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 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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