[linux-lvm] Performance problem.

[Henric Andersson]

> dd_rescue /dev/vg_main/storagelv /dev/null
>
> dd_rescue: (info): ipos:    138384.0k, opos:    138384.0k, xferd:
138384.0k
>                    errs:      0, errxfer:         0.0k, succxfer:
138384.0k
>              +curr.rate:    27966kB/s, avg.rate:    27553kB/s, avg.load:
20.9%

This proves nothing I'm afraid. I've done similar tests myself and even
gotten bonnie to produce results that "look good", but these are all
synthetic benchmarks. What counts is realworld performance. I tested bonnie
(locally on lvm and regular partitions) and regular filecopy with time
(using a 512MB file) between LVM to the system drive. Also tried NFS
compared to Samba and it made no difference. FTP proved the same. At the
beginning of all the test where I could see the actual speed, it would be
right on spot (ie, maxing out my 100mbit switched network) but after 10
seconds or so, the speed would drop dramatically to 2-4mb/s (from ~9mb/s)
and then even halt completely for a short while and then resuming at 2-4mb/s
(and this with no other network/disk access on the server). Oh, and to
eliminate the destination drive completely during tests, I also tried "time
cat big512mbfile > /dev/null", which, ofcourse, resulted in the same speed
as every other test I did.

This is starting to become a pain. For example, I'm unable to burn my backup
tarfiles to DVD over the network without empting the buffer (thank good for
powerburn) which is ridiculous since all it requires is a consistant 4mb/s
or so. I'm going to try the devicemapper in 2.6 (LVM2), but I haven't had
the time to do it yet (have to create a similar setup since I dont want to
risk the data for a little test).

Oh, and before you ask, the server is a Celeron at 2Ghz, with 512MB memory, all
filesystems are ext3, all drives are master and use the correct ATA/133
cable. All drives use dma and 32bit with sync transfers. The server's sole
purpose in life is to provide storage for my network. It uses an nice Intel
Pro/100 card to ensure stability and performance. The network is 100mbit
full duplex switched to allow max speed. When transfering files from a
normal partition (ie, /dev/hdax) I get about 9-9.5mb/s with samba and
Win2kPro (measured with the Kerio Firewall on the Win2k and IpTraf locally
on the fileserver).

(The serverhardware was purchased since I figured that my previous 120Mhz
Pentium Classic was the bottleneck  :-/ )

/Henric