[CentOS] Strangely slow disk

Just check smart table:

smartctl -a -d ata /dev/sdX

Sometimes disk work very slowly without any visible reason. In this
case just replace your disk.

Check your bios, and make sure that you did not have IDE mode enabled and
AHCI is selected .

Also, is this one of the "Green" series of WD disks? Those have
a 4KB sector size, not the old/traditional 512B sector size. This has
performance ramifications in that you must have partitions begin on
a sector boundary or performance goes to the dogs. There are a number
of articles about it on various geek sites, sorry I can't point you
to one offhand.

many newer disks use this same 4k internal sectoring. the drives still
*look* like they have 512 byte sectors to the outside world, but its
very important for performance to get your partitions properly aligned
on a 4k boundary, and ideally your file system uses 4K (or multiple of
4k) as its block size.

sadly, the default fake CHS (cylinder head sector) mapping left over
from legacy PC architectures ends up with a 255 512b sector 'track', and
MBR partitioning defaults to using track or cylinder boundaries, so the
first partition starts at an odd location if you use legacy tools like
fdisk.

The WDxxEADS drives have 512B sectors, _not_ 4KB sectors.

Thanks for your response, and for the others received.

But I should have pointed out that the slow operation
has only started recently, in the last week or so,
after working normally since I acquired the disk a year or so ago.

To add to the mystery, I just checked with hdparm,
and the speed of the disk (as measured this way) seems normal,
compared with various other disks:
-----------------------------------
The "slow disk"
[tim at helen ~]$ sudo hdparm -t /dev/sdb
/dev/sdb:
Timing buffered disk reads: 168 MB in 3.00 seconds = 55.98 MB/sec
-----------------------------------
Another disk on the same machine
[tim at blanche ~]$ sudo hdparm -t /dev/sda
/dev/sda:
Timing buffered disk reads: 146 MB in 3.01 seconds = 48.57 MB/sec
-----------------------------------
An external hard disk attached to the same machine
[tim at helen ~]$ sudo hdparm -t /dev/sdc

/dev/sdc:
Timing buffered disk reads: 54 MB in 3.03 seconds = 17.80 MB/sec
-----------------------------------

I have Windows XP on another partition.
I don't think I've ever used it
(I always install Windows if it is not already on the machine,
as I've had a couple of occasions when it has saved my life).
I'll see how fast (or slow) Windows backup goes.

Those are slowish times even for a 7200rpm disk. My desktop here at
home (Ubuntu) has a slow 7200 drive and hdparam reports a lot faster
than that. Well, it is a Caviar "Green" drive which means that 7200
is the fastest speed but it does spin slower too.

amckay at amckay-desktop:~$ sudo !!
sudo hdparm -t /dev/sda
[sudo] password for amckay:

/dev/sda:
Timing buffered disk reads: 244 MB in 3.01 seconds = 81.10 MB/sec
amckay at amckay-desktop:~$

You have not said anything yet in this thread about defragging that drive.

I just checked your original message and your drive is the exact same
as mine except yours is the 1.5 TB version and mine is 1.0.

Also - boot a live Linux CD and then from there do hdparam again and
compare results, If they differ vastly at least you know it is
something in your running system which is the culprit. If they are
roughly the same then it is likely the drive gone bad. Though check
the man page for hdparam to see if disk fragmentation will affect the
results it gives.

Here are my?results with a PATA drive and a SATA drive:

[root at poontang ~]# hdparm -t /dev/hda

/dev/hda:
Timing buffered disk reads: 172 MB in 3.02 seconds = 56.97 MB/sec
[root at poontang ~]# hdparm -t /dev/sda

/dev/sda:
Timing buffered disk reads: 300 MB in 3.01 seconds = 99.66 MB/sec

Your results are similar to what I get with my PATA drive, so I agree
with Ho Chaw Ming that you should check that AHCI is selected.