Extremely slow drive despite lack of errors


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I have a samsung (HD203WI) that I've isolated as the source for some very slow parity checks. Previously I was getting 90MB/sec+ parity checks on a 12 disk + parity system, but the past few days have been around 23MB/sec.

 

Nothing showed up in syslog and the SMART tests all passed. However, when I run hdparm I get some interesting results:

 

root@Hal:~# hdparm -tT /dev/sdk

/dev/sdk:

Timing cached reads:  19782 MB in  2.00 seconds = 9915.27 MB/sec

Timing buffered disk reads:  28 MB in  3.10 seconds =  9.02 MB/sec

 

root@Hal:~# hdparm -tT /dev/sdl

/dev/sdl:

Timing cached reads:  21672 MB in  1.99 seconds = 10864.71 MB/sec

Timing buffered disk reads: 280 MB in  3.00 seconds =  93.26 MB/sec

 

/dev/sdk is the Samsung drive which just tested @ 1.68 MB/sec and is going to be replaced. It is definitely the drive as I've moved another drive to its slot and it tests fine, move the Samsung to a known good port/cable and it remains slow.

 

I have actually seen this once before in an iMac... drive performance was abysmal yet every hardware test passed with flying colors.

 

Once I replace the drive, I'm going to setup automated hdparm tests once a week in case this happens again.

 

But I am really curious what causes this when all tests pass and nothing gets logged. Any ideas?

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