[mythtv-users] SATA Drive Failure

Rudy Zijlstra mythtv at edsons.demon.nl
Sat Jun 25 12:29:42 UTC 2005


Greg wrote:

>>Ummm... No. You're reading the results wrongly, I'm afraid :) 
>>I would seriously doubt you'd see 1.5Gb / sec outside the datacentre.
>>150Mb / sec... _maybe_.
>>
>>Cached reads are just that - It reads the same sector over and over. 
>>This means it's already in drive's read cache. So what you're 
>>seeing here is the transfer rate from the controller without 
>>(much) SATA latency, no seek latency, no rotational latency. 
>>That's not the test you want. You want 'buffered disk reads'. 
>>This is the (almost real) raw performance of the drive.
>>
>>Here's the results from my array, which is a 8 disk RAID5, 
>>spread over 2 SATA controllers on a PCI-X dual Xeon (yes, I 
>>believe overkill _is_ just a word):
>>
>>hdparm -Tt /dev/sda
>>
>>/dev/sda:
>>  Timing cached reads:   2204 MB in  2.00 seconds = 1101.07 MB/sec
>>  Timing buffered disk reads:  374 MB in  3.01 seconds = 124.31 MB/sec
>>
>>The 'buffered disk reads' is the line you're looking for. And 
>>124Mb / sec is quite good for this type of setup.
>>
>>Cheers,
>>
>>Allan.
>>    
>>
>
>Allan, 
>
>Isnt sda just one of the disks in your array??
>I get:
>
>Hdparm -Tt /dev/hda (single 40gb pata)
>	Timing cached reads:   2396 MB in  2.00 seconds = 1196.98 MB/sec
>	Timing buffered disk reads:  122 MB in  3.04 seconds = 40.14 MB/sec
>
>Hdparm -Tt /dev/sda (160gb sata, 1st disk in 2 disk software raid)
>	Timing cached reads:   2412 MB in  2.00 seconds = 1204.98 MB/sec
>	Timing buffered disk reads:  110 MB in  3.02 seconds = 36.39 MB/sec
>
>Hdparm -Tt /dev/md0 (60gb raid1)
>	Timing cached reads:   2424 MB in  2.00 seconds = 1210.97 MB/sec
>	Timing buffered disk reads:  110 MB in  3.02 seconds = 36.38 MB/sec
>
>Hdparm -Tt /dev/md1 (200gb raid0)
>	Timing cached reads:   2408 MB in  2.00 seconds = 1201.78 MB/sec
>	Timing buffered disk reads:  256 MB in  3.08 seconds = 83.08 MB/sec
>
>Greg
>
>  
>
Greg,

I am getting on a HW SCSI Raid system:

/dev/sdd:
 Timing cached reads:   2080 MB in  2.00 seconds = 1038.60 MB/sec
 Timing buffered disk reads:  190 MB in  3.01 seconds =  63.03 MB/sec

On this system i get about the same performance, no matter which raid 
set i test - limitation seems to be the controller.

on a SW raid5 over 4 disks (3/4 Tb)
/dev/md0:
 Timing cached reads:   1440 MB in  2.00 seconds = 719.03 MB/sec
 Timing buffered disk reads:  116 MB in  3.04 seconds =  38.18 MB/sec

Cheers,

Rudy



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