I am trying to tune my NAS, running openfiler, and wondering why I'm getting relatively poor read performance from 4 WD RE3 drives in RAID 5.
EDIT: Please note I am talking about the buffered disk read speed not cached speeds
EDIT: Changed formatting to make clear there are two sets of output.
When I run hdparm on the meta device I get the levels of performance I'd expect, drop to the volume and it's a third the speed !
Any one any idea why ? Is LVM that bad ?
Meta device /dev/md0 results
[root@nas2 etc]# hdparm -tT /dev/md0 /dev/md0: Timing cached reads: 4636 MB in 2.00 seconds = 2318.96 MB/sec Timing buffered disk reads: 524 MB in 3.01 seconds = 174.04 MB/sec
Vol group /dev/mapper/vg1-vol1 results
[root@nas2 etc]# hdparm -tT /dev/mapper/vg1-vol1 /dev/mapper/vg1-vol1: Timing cached reads: 4640 MB in 2.00 seconds = 2320.28 MB/sec Timing buffered disk reads: 200 MB in 3.01 seconds = 66.43 MB/sec
Edit: See section from the hdparm man page which suggest this is perfectly valid test for sequential read performance which is the issue I am trying to resolve.
-t Perform timings of device reads for benchmark and comparison purposes. For meaningful results, this operation should be repeated 2-3 times on an otherwise inactive system (no other active processes) with at least a couple of megabytes of free memory. This displays the speed of reading through the buffer cache to the disk without any prior caching of data. This measurement is an indication of how fast the drive can sustain sequential data reads under Linux, without any filesystem overhead. To ensure accurate measurements, the buffer cache is flushed during the processing of -t using the BLKFLSBUF ioctl. If the -T flag is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t operation.