[CentOS] One disk speed problem [SOLVED], and a question on hdparm

m.roth at 5-cent.us m.roth at 5-cent.us
Mon Mar 26 16:49:21 EDT 2012


I believe I've posted before about one of the speed issues we were having,
of backups taking many, many hours that should *not* take that long.

My manager and I finally nailed it down to the h/d itself. Identical
boxes, and he tried a backup of one system which took under two hours,
while the same regular one rand nearly six.

I'd been googling on and off for weeks, and this morning, ran across
something: partition alignment. A thread where someone who'd done some
tests and found that was his problem.

So, here's the answer: we have these Caviar Green 2tb drives (the thread I
found had either a 1tb, or 1.5tb), (and no, we are *NOT* going to buy
Caviar Green for servers ever again). The big thing is that they use 4k
sectors, *not* 512 bytes. Following directions, I pulled it into fdisk,
and then used a command I've not needed before: u. This changes units from
cylinders (the default) to sectors. Having done that, p shows that it
actually starts in sector 63. Again, following directions, I changed it to
start in sector 64. Finished the partition, wrote it, made the filesystem,
and tried it out.

177G transferred in 1h 47+m.

So, anyone who's got new drives that use 4k sectors should probably follow
this.

This also probably explains parted's completely aggravating complaint that
the partition's not aligned, but gives you no idea *why*, or how to align
it - I pulled the drive into parted, and told it to print, and it did
*not* complain the partition wasn't aligned.

Next trick: hdparm. I want to disable, or at least shove way up, the
spindown timeout on these drives, which, depending on the thread you read,
is 6 or 8 seconds. hdparm should let me do that... but before I do, I'd
like to know what it's set at. Does anyone know how to find that out? And
no, the manpage is wrong, hdparm -B <no parm) /dev/sdx does *not* read it,
it just complains it's missing the parm.

         mark



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