Messages in this thread | | | Date | Tue, 31 Aug 2004 19:00:16 +0200 | From | Erik Mouw <> | Subject | Re: Driver retries disk errors. |
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On Tue, Aug 31, 2004 at 04:13:54PM +0100, Alan Cox wrote: > On Maw, 2004-08-31 at 16:56, Erik Mouw wrote: > > The SCSI disk driver has been doing a single retry for quite some time > > and it hasn't really bitten people. Why would the IDE disk driver be > > different? The only case I can imagine a retry would be OK, is when we > > get an UDMA CRC error (caused by bad cables). > > Retries also pop up in other less obvious cases and conveniently paper > over a wide variety of timeouts, power management quirks and drives just > having a random fit. Eight is probably excessive in all cases.
There are indeed all sorts of other retries in various layers, the worst one when the kernel tries to read-ahead a couple of blocks on an IDE disk(1). You can work around them with raw IO or O_DIRECT.
> For non hard disk cases many devices do want and need retry.
And many others do not. CompactFlash readers are usually implemented as a USB storage device, which on its turn is implemented as a SCSI "disk". So far I haven't seen a CompactFlash which could be "fixed" by retries.
iSCSI appliances can also make things worse: when the target machine is implemented as a simple "pass everything to the real SCSI disk" device, it's not really different from a directly connected SCSI disk. However, when the iSCSI target interprets the SCSI commands and has its own way to deal with bad blocks (i.e.: it retries the blocks), things can get very bad when the kernel also uses a couple of retries.
In my experience it would be good if the IDE disk driver would behave like the SCSI disk driver: no retries on a bad block. I agree that it would be a good idea to make it configurable through the block layer to avoid code duplication (blockdev --getretries/--setretries).
Erik
(1) Imagine an application doing a linear read on a file with an 8 block read ahead and the last block being bad. The kernel will try to read that bad block 16 times, but because the IDE driver also has 8 retries, the kernel will try to read that bad block *64* times. It usually takes an IDE drive about 2 seconds to figure out a block is bad, so the application gets stuck for 2 minutes in that single bad block.
-- +-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 -- | Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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