On Wed, Dec 9, 2009 at 3:30 AM, hank peng <pengxihan@xxxxxxxxx> wrote: > 2009/12/9 Michael Evans <mjevans1983@xxxxxxxxx>: >> On Tue, Dec 8, 2009 at 6:03 AM, hank peng <pengxihan@xxxxxxxxx> wrote: >>> 2009/12/8 Robin Hill <robin@xxxxxxxxxxxxxxx>: >>>> On Tue Dec 08, 2009 at 09:49:48PM +0800, hank peng wrote: >>>> >>>>> 2009/12/8 Robin Hill <robin@xxxxxxxxxxxxxxx>: >>>>> > On Tue Dec 08, 2009 at 09:01:23PM +0800, hank peng wrote: >>>>> > >>>>> >> Hi, all: >>>>> >> As we know, when a raid5 array is created, recovery will be going on >>>>> >> which involves some read, one xor and one write. Since there is no >>>>> >> real data in the disk at the time, besides, if I am willing to wait >>>>> >> for recovery to complete and then use this raid5, how about adding >>>>> >> support for a fast recovery method? Right now, what is in my mind is >>>>> >> zero all disks which belong to this raid5. I think it will increase >>>>> >> raid5 recovery speed when created and decrease CPU usage, since all >>>>> >> zero is also XORed. >>>>> >> What do raid developers think? >>>>> >> >>>>> > It'll decrease CPU usage but increase I/O - you're now needing to write >>>>> > to all disks. Most systems will be I/O limited rather than CPU limited, >>>>> > so the current approach works better. If you want to zero the disks >>>>> > then do this before creating the array - you can then use --assume-clean >>>>> > to skip the resync process. >>>>> > >>>>> I think --assume-clean is used mostly when doing performance test and >>>>> can't be used when creating a raid5 array using new disk, because >>>>> later read and write operation make assumption that all stripe is >>>>> XORed. Correct me if I am wrong. >>>>> >>>> You're correct - that's why I said to zero all the disks first so the >>>> XOR data is all correct. >>>> >>> I think this function is better to be implemented in kernel raid >>> layer, not in user space(for example using dd command). >>> In this way, we can get good performance and lower cpu usage, also, we >>> can make this function be part of raid code so that it can be managed >>> by mdadm >>>> Cheers, >>>> Robin >>>> -- >>>> ___ >>>> ( ' } | Robin Hill <robin@xxxxxxxxxxxxxxx> | >>>> / / ) | Little Jim says .... | >>>> // !! | "He fallen in de water !!" | >>>> >>> >>> >>> >>> -- >>> The simplest is not all best but the best is surely the simplest! >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> >> >> How about documenting this better? 'zeroing all underlying devices >'Lost in translation' >> then creating with --assume-clean' will be clean because the parity >> algorithm is even (or similar to 'even parity')? >> -- Maybe this will translate more easily. The documentation should be more explicit. "When the devices the RAID is made of are filled with zero's before RAID creation --assume-clean can be used because the parity will already be correct." -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
