I have an existing RAID Array and I want to increase the readahead setting. But the device has already been formatted and contains data that I cannot afford to lose. I'm unfamiliar with the blockdev command. Is it safe to run blockdev –setra=xxxxx on devices that have already been formatted? Will running the following command destroy (delete) the data on my device? Is there any risk of this happening?
$ blockdev --setra 65536 /dev/md0
Not sure this is important, but here are the devices details:
$ xfs_info /data/d1
meta-data=/dev/md0 isize=256 agcount=16, agsize=8191936 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=131070976, imaxpct=25
= sunit=64 swidth=256 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal bsize=4096 blocks=64000, version=2
= sectsz=512 sunit=64 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
$ sudo mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Thu May 5 17:23:35 2011
Raid Level : raid0
Array Size : 524283904 (500.00 GiB 536.87 GB)
Raid Devices : 4
Total Devices : 4
Persistence : Superblock is persistent
Update Time : Thu May 5 17:23:35 2011
State : clean
Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0
Chunk Size : 256K
Name : d1:0
UUID : 16d7fee4:a8903d2f:28c65d2f:161006b5
Events : 0
Number Major Minor RaidDevice State
0 202 241 0 active sync /dev/sdp1
1 202 242 1 active sync /dev/sdp2
2 202 243 2 active sync /dev/sdp3
3 202 244 3 active sync /dev/sdp4
Best Answer
"blockdev --setra" is perfectly safe. We have it in our init scripts to run on every boot on a couple of servers. You probably want to do the same after you have benchmarked a few values and decided which is best for your workload.