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Re: Reasonable maximum package size ?

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On 11 Jun 2007, at 9:22 pm, Josselin Mouette wrote:

You seem to strongly believe the cheap desktop hard disk is different
from the server hard disk. This is entirely wrong. Apart from 10k and
15k rpm disks, these are all strictly the same. Only the electronics

That's not true, unfortunately. They also have different design criteria for duty cycles, and more stringent MTBF testing requirements. There's been a lot of assertion in this thread, without any real data, so this post provides links to some hard data provided by disk manufacturers.

Here follows a posting I sent to the Bioclusters mailing list a few months ago, which includes PDF documents from Seagate showing that their desktop products are absolutely *not* equivalent to their higher-end spindles:

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Practical experience of running a petabyte of storage arrays here is what I'm basing my opinion on, not claims for device MTBF. Besides, MTBF is highly dependent on duty cycle. MTBF figures are meaningless if you don't also consider the duty cycle. You need to make sure the spindles are designed for a 24/7 duty cycle. Cheap SATA drives normally are not.

Anyway, you asked for figures, so here we are, from a spindle manufacturer and (later) from Microsoft (yes, I know):


Their nearline fibrechannel drives have an MTBF of 1 million hours, 24/7 duty cycle, read/write. Their desktop SATA drives have an MTBF of 600,000 hours, but that's for an 8x5 largely read-only duty cycle. If you abuse them by running them 24x7 read-write, I dare say it will be considerably less. But ignoring that, by my rough estimate, a double drive failure causing data loss of your RAID5 array using desktop SATA drives will probably happen about four times more frequently than using fibrechannel disks. Of course, you can buy high MTBF SATA drives but (surprise!) they cost about the same as the fibrechannel ones.

More details here about what Seagate put in their cheap drives vs. the expensive ones:

http://www.seagate.com/content/docs/pdf/whitepaper/ D2c_More_than_Interface_ATA_vs_SCSI_042003.pdf

There are also some sobering graphs in this joint presentation from Seagate and Microsoft (apologies for the Powerpoint):

http://download.microsoft.com/download/9/8/f/98f3fe47- dfc3-4e74-92a3-088782200fe7/TWST05005_WinHEC05.ppt

The graph showing the probability of second disk error during RAID5 rebuild on desktop and server drives is slightly scary even for server drives, but positively terrifying for the cheap drives. This of course, is why we use RAID6 and a hot spare in our large Lustre filesystems. RAID5 is simply not reliable enough, using the SATA drives which underly our SFS servers.

As many others have said in this thread, you get cheap or reliable. You do not get both.

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