Re: Clarifications about check/repair, i.e. RAID SCRUBBING
Thanks for clearing things up, Neil. Looks like I will be issuing weekly "repairs" on most of the arrays. Cheers, -- roy Neil Brown wrote: On Friday June 2, [EMAIL PROTECTED] wrote: In any regard: I'm talking about triggering the following functionality: echo check > /sys/block/mdX/md/sync_action echo repair > /sys/block/mdX/md/sync_action On a RAID5, and soon a RAID6, I'm looking to set up a cron job, and am trying to figure out what exactly to schedule. The answers to the following questions might shed some light on this: 1. GENERALLY SPEAKING, WHAT IS THE DIFFERENCE BETWEEN THE "CHECK" AND "REPAIR" COMMANDS? The "md.txt" doc mentions for "check" that "a repair may also happen for some raid levels." Which RAID levels, and in what cases? If I perform a "check" is there a cache of bad blocks that need to be fixed that can quickly be repaired by executing the "repair" command? Or would it go through the entire array again? I'm working with new drives, and haven't come across any bad blocks to test this with. 'check' just reads everything and doesn't trigger any writes unless a read error is detected, in which case the normally read-error handing kicks in. So it can be useful on a read-only array. 'repair' does that same but when it finds an inconsistency is corrects it by writing something. If any raid personality had not be taught to specifically understand 'check', then a 'check' run would effect a 'repair'. I think 2.6.17 will have all personalities doing the right thing. check doesn't keep a record of problems, just a count. 'repair' will reprocess the whole array. 2. CAN "CHECK" BE RUN ON A DEGRADED ARRAY (say with N out of N+1 disks on a RAID level 5)? I can test this out, but was it designed to do this, versus "REPAIR" only working on a full set of active drives? Perhaps "repair" is assuming that I have N+1 disks so that parity can be WRITTEN? No, check on a degraded raid5, or a raid6 with 2 missing devices, or a raid1 with only one device will not do anything. It will terminate immediately. After all, there is nothing useful that it can do. 3. RE: FEEDBACK/LOGGING: it seems that I might see some messages in dmesg logging output such as "raid5:read error corrected!", is that right? I realize that "mismatch_count" can also be used to see if there was any "action" during a "check" or "repair." I'm assuming this stuff doesn't make its way into an email. You are correct on all counts. mdadm --monitor doesn't know about this yet. ((writes notes in mdadm todo list)). 4. DOES "REPAIR" PERFORM READS TO CHECK THE ARRAY, AND THEN WRITE TO THE ARRAY *ONLY WHEN NECESSARY* TO PERFORM FIXES FOR CERTAIN BLOCKS? (I know, it's sorta a repeat of question number 1+2). repair only writes when necessary. In the normal case, it will only read every blocks. 5. IS THERE ILL-EFFECT TO STOP EITHER "CHECK" OR "REPAIR" BY ISSUING "IDLE"? No. 6. IS IT AT ALL POSSIBLE TO CHECK A CERTAIN RANGE OF BLOCKS? And to keep track of which blocks were checked? The motivation is to start checking some blocks overnight, and to pick-up where I left off the next night... Not yet. It might be possible one day. 7. ANY OTHER CONSIDERATIONS WHEN "SCRUBBING" THE RAID? Not that I am aware of. NeilBrown Sorry for some of these questions being so similar in nature. I just want to make sure I understand it correctly. Neil, again, a BIG thanks for this new functionality. I'm looking forward to putting a system in place to exercise my drives! Cheers, -- roy - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Clarifications about check/repair, i.e. RAID SCRUBBING
On Friday June 2, [EMAIL PROTECTED] wrote: > > In any regard: > > I'm talking about triggering the following functionality: > > echo check > /sys/block/mdX/md/sync_action > echo repair > /sys/block/mdX/md/sync_action > > On a RAID5, and soon a RAID6, I'm looking to set up a cron job, and am > trying to figure out what exactly to schedule. The answers to the > following questions might shed some light on this: > > 1. GENERALLY SPEAKING, WHAT IS THE DIFFERENCE BETWEEN THE "CHECK" AND > "REPAIR" COMMANDS? > The "md.txt" doc mentions for "check" that "a repair may also happen for > some raid levels." > Which RAID levels, and in what cases? If I perform a "check" is there a > cache of bad blocks that need to be fixed that can quickly be repaired > by executing the "repair" command? Or would it go through the entire > array again? I'm working with new drives, and haven't come across any > bad blocks to test this with. 'check' just reads everything and doesn't trigger any writes unless a read error is detected, in which case the normally read-error handing kicks in. So it can be useful on a read-only array. 'repair' does that same but when it finds an inconsistency is corrects it by writing something. If any raid personality had not be taught to specifically understand 'check', then a 'check' run would effect a 'repair'. I think 2.6.17 will have all personalities doing the right thing. check doesn't keep a record of problems, just a count. 'repair' will reprocess the whole array. > > 2. CAN "CHECK" BE RUN ON A DEGRADED ARRAY (say with N out of N+1 disks > on a RAID level 5)? I can test this out, but was it designed to do > this, versus "REPAIR" only working on a full set of active drives? > Perhaps "repair" is assuming that I have N+1 disks so that parity can be > WRITTEN? No, check on a degraded raid5, or a raid6 with 2 missing devices, or a raid1 with only one device will not do anything. It will terminate immediately. After all, there is nothing useful that it can do. > > 3. RE: FEEDBACK/LOGGING: it seems that I might see some messages in > dmesg logging output such as "raid5:read error corrected!", is that > right? I realize that "mismatch_count" can also be used to see if there > was any "action" during a "check" or "repair." I'm assuming this stuff > doesn't make its way into an email. You are correct on all counts. mdadm --monitor doesn't know about this yet. ((writes notes in mdadm todo list)). > > 4. DOES "REPAIR" PERFORM READS TO CHECK THE ARRAY, AND THEN WRITE TO THE > ARRAY *ONLY WHEN NECESSARY* TO PERFORM FIXES FOR CERTAIN BLOCKS? (I > know, it's sorta a repeat of question number 1+2). > repair only writes when necessary. In the normal case, it will only read every blocks. > 5. IS THERE ILL-EFFECT TO STOP EITHER "CHECK" OR "REPAIR" BY ISSUING "IDLE"? No. > > 6. IS IT AT ALL POSSIBLE TO CHECK A CERTAIN RANGE OF BLOCKS? And to > keep track of which blocks were checked? The motivation is to start > checking some blocks overnight, and to pick-up where I left off the next > night... Not yet. It might be possible one day. > > 7. ANY OTHER CONSIDERATIONS WHEN "SCRUBBING" THE RAID? > Not that I am aware of. NeilBrown > Sorry for some of these questions being so similar in nature. I just > want to make sure I understand it correctly. > > Neil, again, a BIG thanks for this new functionality. I'm looking > forward to putting a system in place to exercise my drives! > > Cheers, > > -- roy > - > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to [EMAIL PROTECTED] > More majordomo info at http://vger.kernel.org/majordomo-info.html - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
Clarifications about check/repair, i.e. RAID SCRUBBING
Hi Neil/folks, I'm seeking some (hopefully) simple clarifications about the newer raid checking and scrubbing behavior present in more recent kernels. I must say that I was more than pleased when I learned about the new functionality. Kudos, Neil for this addition. Unfortunately, because this is new it's not to be found in the FAQs or HOW-TOs... with the exception of the Gentoo "HOWTO Install on Software RAID". I've looked at the following sources of info: linux-2.6.16.19/Documentation/md.txt linux-2.6.16.19/drivers/md:raid5.c and raid6main.c (the raid5_end_read_request and raid6_end_read_request routines) emails on the linux-raid mailing list, in particular: http://lkml.org/lkml/2005/12/4/118 http://www.mail-archive.com/linux-raid@vger.kernel.org/msg04615.html === In any regard: I'm talking about triggering the following functionality: echo check > /sys/block/mdX/md/sync_action echo repair > /sys/block/mdX/md/sync_action On a RAID5, and soon a RAID6, I'm looking to set up a cron job, and am trying to figure out what exactly to schedule. The answers to the following questions might shed some light on this: 1. GENERALLY SPEAKING, WHAT IS THE DIFFERENCE BETWEEN THE "CHECK" AND "REPAIR" COMMANDS? The "md.txt" doc mentions for "check" that "a repair may also happen for some raid levels." Which RAID levels, and in what cases? If I perform a "check" is there a cache of bad blocks that need to be fixed that can quickly be repaired by executing the "repair" command? Or would it go through the entire array again? I'm working with new drives, and haven't come across any bad blocks to test this with. 2. CAN "CHECK" BE RUN ON A DEGRADED ARRAY (say with N out of N+1 disks on a RAID level 5)? I can test this out, but was it designed to do this, versus "REPAIR" only working on a full set of active drives? Perhaps "repair" is assuming that I have N+1 disks so that parity can be WRITTEN? 3. RE: FEEDBACK/LOGGING: it seems that I might see some messages in dmesg logging output such as "raid5:read error corrected!", is that right? I realize that "mismatch_count" can also be used to see if there was any "action" during a "check" or "repair." I'm assuming this stuff doesn't make its way into an email. 4. DOES "REPAIR" PERFORM READS TO CHECK THE ARRAY, AND THEN WRITE TO THE ARRAY *ONLY WHEN NECESSARY* TO PERFORM FIXES FOR CERTAIN BLOCKS? (I know, it's sorta a repeat of question number 1+2). 5. IS THERE ILL-EFFECT TO STOP EITHER "CHECK" OR "REPAIR" BY ISSUING "IDLE"? 6. IS IT AT ALL POSSIBLE TO CHECK A CERTAIN RANGE OF BLOCKS? And to keep track of which blocks were checked? The motivation is to start checking some blocks overnight, and to pick-up where I left off the next night... 7. ANY OTHER CONSIDERATIONS WHEN "SCRUBBING" THE RAID? Sorry for some of these questions being so similar in nature. I just want to make sure I understand it correctly. Neil, again, a BIG thanks for this new functionality. I'm looking forward to putting a system in place to exercise my drives! Cheers, -- roy - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
