Re: RAID10 Balancing Request for Comments and Advices
Vincent Olivier posted on Wed, 17 Jun 2015 09:46:50 -0400 as excerpted: On Jun 16, 2015, at 7:58 PM, Duncan 1i5t5.dun...@cox.net wrote: Yes. GlobalReserve is for short-term btrfs-internal use, reserved for times when btrfs needs to (temporarily) allocate some space in ordered to free space, etc. It's always single, and you'll rarely see anything but 0 used except perhaps in the middle of a balance or something. Get it. Thanks. Is there anyway to put that on another device, say, a SSD? Not (AFAIK) presently. There are various btrfs feature suggestions involving selective steering various btrfs component bits to faster or slower devices, etc, as can be seen on the wiki, but the btrfs chunk allocator isn't really customizable beyond basic raid-level, yet. It does what it does and that's it. For fancy features such as this, unless you're a company or individual with resources to invest in a specific feature of interest, I'd say give btrfs development another five years or so, and it may be tackling this sort of thing. The two actually working alternatives I know of are bcached btrfs (there's someone on-list that actually does that and reports it working), and a more mature btrfs-similar solution such as zfs, tho of course zfs on Linux has its own issues, primarily licensing/legal. I am thinking of backing up this RAID10 on a 2x8TB device-managed SMR RAID1 and I want to minimize random write operations (noatime al.). I will start a new thread for that maybe but first, is there something substantial I can read about btrfs+SMR? Or should I avoid SMR+btfs ? I haven't the foggiest, but in case it spares someone looking up SMR like I just had to do, SMR = Shingled Magnetic Recording -- the new shingled drives that have been in the tech news since shortly before they started shipping in late 2013. https://en.wikipedia.org/wiki/Shingled_magnetic_recording ok then, rule of the thumb re-run the scrub on “unverified checksum error(s)”. I have yet to see checksum errors yet but will keep it in mind.. FWIW, see my few minutes ago reply to Marc MERLIN in the BTRFS: read error corrected: ino 1 off thread, if you're interested in further discussion on this. But regardless, based on my own experience, that's a good rule of thumb, yes. =:^) Meanwhile, I'm having a bit of morbid fun watching as [a dying ssd] slowly decays, getting experience of the process in a reasonably controlled setting without serious danger to my data, since it is backed up. You sure have morbid inclinations ! ;-) =:^) Out of curiosity what is the frequency and sequence of smartctl long/short tests + btrfs scrubs ? Is it all automated ? I haven't automated any of that, except that since this dying ssd thing started I created a small scriptlet (could be an alias, but I prefer scriptlets), bscrub, that runs btrfs scrub start -Bd $*, to avoid typing in the full command. All I have to add is the mountpoint to scrub, possibly preceded by -r to read-only scrub /, which I keep read- only mounted by default. Perhaps to my harm I don't actually do the smart-tests regularly. I'm not actually sure they're particularly useful on SSDs, particularly when using checksum-verified and raid-redundant filesystems such as btrfs in raid1/10 mode (and raid5/6 as it matures). In practice btrfs scrub regularly reporting error corrected and/or nasty bus reset errors showing up in the logs are a pretty good advance indicators, better than smart status, from what I've seen. I do check smartctrl -AH regularly, particularly now, but (in the past at least, I think my habit may be changing for the better, now, one of the positive results of letting the dying ssd run for the moment) less frequently when no problems are evident. I actually have a pretty firm policy of splitting up my data onto separate filesystems (btrfs subvolumes don't cut it for me as all the data eggs are still in the same filesystem basket and if its bottom falls out, ), keeping them of easily managed and easily backed up size. My largest btrfs is actually under 50 gig. Between that and the fact that I'm using ssds, whole-filesystem maintenance (btrfs scrub, balance, and check commands) time is on the order of seconds to a few minutes (single digits) per filesystem. As a result, running them is relatively trivial -- it doesn't take the hours to days people report for their multi- terabyte btrfs on spinning rust, and I can and do sometimes run them on a whim. Scrubs are generally under a minute per filesystem, with only a handful of filesystems routinely used, so under 10 minutes, total, including repeat-runs, on all routinely mounted btrfs. Given the trivial time factor I basically simply integrated the scrub into my update procedure (weekly on average, tho it can be daily if I'm waiting on a fix or 10-14 days if I'm lazy), since that's my biggest filesystem changes and thus most likely to trigger new
Re: RAID10 Balancing Request for Comments and Advices
Hugo Mills posted on Wed, 17 Jun 2015 13:27:36 + as excerpted: Yes, on this 80% full 6x4TB RAID10 -dusage=15 took 2 seconds and relocated 0 out of 3026 chunks”. Out of curiosity, I had to use -dusage=90 to have it relocate only 1 chunk and it took les than 30 seconds. So I put a -dusage=25 in the weekly cron just before the scrub. In most cases, all you need to do is clean up one data chunk to give the metadata enough space to work in. Instead of manually iterating through several values of usage= until you get a useful response, you can use limit=n to stop after n successful block group relocations. Thanks, Hugo. It wasn't previously clear to me what the practical usage for the (relatively new) limit= filter was. Very useful explanation. =:^) -- Duncan - List replies preferred. No HTML msgs. Every nonfree program has a lord, a master -- and if you use the program, he is your master. Richard Stallman -- To unsubscribe from this list: send the line unsubscribe linux-btrfs in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: RAID10 Balancing Request for Comments and Advices
On Jun 17, 2015, at 9:27 AM, Hugo Mills h...@carfax.org.uk wrote: On Wed, Jun 17, 2015 at 09:13:08AM -0400, Vincent Olivier wrote: On Jun 16, 2015, at 8:14 PM, Chris Murphy li...@colorremedies.com wrote: On Tue, Jun 16, 2015 at 5:58 PM, Duncan 1i5t5.dun...@cox.net wrote: On a current kernel unlike older ones, btrfs actually automates entirely empty chunk reclaim, so this problem doesn't occur anything close to near as often as it used to. However, it's still possible to have mostly but not entirely empty chunks that btrfs won't automatically reclaim. A balance can be used to rewrite and combine these mostly empty chunks, reclaiming the space saved. This is what Hugo was recommending. Yes, as little as a -dusage=5 (data chunks that are 5% or less full) can clear the problem and is very fast, seconds. Possibly a bit longer, many seconds o single digit minutes is -dusage=15. I haven't done a full balance in forever. Yes, on this 80% full 6x4TB RAID10 -dusage=15 took 2 seconds and relocated 0 out of 3026 chunks”. Out of curiosity, I had to use -dusage=90 to have it relocate only 1 chunk and it took les than 30 seconds. So I put a -dusage=25 in the weekly cron just before the scrub. In most cases, all you need to do is clean up one data chunk to give the metadata enough space to work in. Instead of manually iterating through several values of usage= until you get a useful response, you can use limit=n to stop after n successful block group relocations. Nice! Will do that instead! Thanks. signature.asc Description: Message signed with OpenPGP using GPGMail
Re: RAID10 Balancing Request for Comments and Advices
On Jun 16, 2015, at 8:14 PM, Chris Murphy li...@colorremedies.com wrote: On Tue, Jun 16, 2015 at 5:58 PM, Duncan 1i5t5.dun...@cox.net wrote: On a current kernel unlike older ones, btrfs actually automates entirely empty chunk reclaim, so this problem doesn't occur anything close to near as often as it used to. However, it's still possible to have mostly but not entirely empty chunks that btrfs won't automatically reclaim. A balance can be used to rewrite and combine these mostly empty chunks, reclaiming the space saved. This is what Hugo was recommending. Yes, as little as a -dusage=5 (data chunks that are 5% or less full) can clear the problem and is very fast, seconds. Possibly a bit longer, many seconds o single digit minutes is -dusage=15. I haven't done a full balance in forever. Yes, on this 80% full 6x4TB RAID10 -dusage=15 took 2 seconds and relocated 0 out of 3026 chunks”. Out of curiosity, I had to use -dusage=90 to have it relocate only 1 chunk and it took les than 30 seconds. So I put a -dusage=25 in the weekly cron just before the scrub. FYI. Thanks for your help.-- To unsubscribe from this list: send the line unsubscribe linux-btrfs in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: RAID10 Balancing Request for Comments and Advices
On Wed, Jun 17, 2015 at 09:13:08AM -0400, Vincent Olivier wrote: On Jun 16, 2015, at 8:14 PM, Chris Murphy li...@colorremedies.com wrote: On Tue, Jun 16, 2015 at 5:58 PM, Duncan 1i5t5.dun...@cox.net wrote: On a current kernel unlike older ones, btrfs actually automates entirely empty chunk reclaim, so this problem doesn't occur anything close to near as often as it used to. However, it's still possible to have mostly but not entirely empty chunks that btrfs won't automatically reclaim. A balance can be used to rewrite and combine these mostly empty chunks, reclaiming the space saved. This is what Hugo was recommending. Yes, as little as a -dusage=5 (data chunks that are 5% or less full) can clear the problem and is very fast, seconds. Possibly a bit longer, many seconds o single digit minutes is -dusage=15. I haven't done a full balance in forever. Yes, on this 80% full 6x4TB RAID10 -dusage=15 took 2 seconds and relocated 0 out of 3026 chunks”. Out of curiosity, I had to use -dusage=90 to have it relocate only 1 chunk and it took les than 30 seconds. So I put a -dusage=25 in the weekly cron just before the scrub. In most cases, all you need to do is clean up one data chunk to give the metadata enough space to work in. Instead of manually iterating through several values of usage= until you get a useful response, you can use limit=n to stop after n successful block group relocations. Hugo. -- Hugo Mills | Alert status mauve ocelot: Slight chance of hugo@... carfax.org.uk | brimstone. Be prepared to make a nice cup of tea. http://carfax.org.uk/ | PGP: E2AB1DE4 | signature.asc Description: Digital signature
Re: RAID10 Balancing Request for Comments and Advices
On Jun 16, 2015, at 7:58 PM, Duncan 1i5t5.dun...@cox.net wrote: Vincent Olivier posted on Tue, 16 Jun 2015 09:34:29 -0400 as excerpted: On Jun 16, 2015, at 8:25 AM, Hugo Mills h...@carfax.org.uk wrote: On Tue, Jun 16, 2015 at 08:09:17AM -0400, Vincent Olivier wrote: My first question is this : is it normal to have “single” blocks ? Why not only RAID10? I don’t remember the exact mkfs options I used but I certainly didn’t ask for “single” so this is unexpected. Yes. It's an artefact of the way that mkfs works. If you run a balance on those chunks, they'll go away. (btrfs balance start -dusage=0 -musage=0 /mountpoint) Thanks! I did and it did go away, except for the GlobalReserve, single: total=512.00MiB, used=0.00B”. But I suppose this is a permanent fixture, right? Yes. GlobalReserve is for short-term btrfs-internal use, reserved for times when btrfs needs to (temporarily) allocate some space in ordered to free space, etc. It's always single, and you'll rarely see anything but 0 used except perhaps in the middle of a balance or something. Get it. Thanks. Is there anyway to put that on another device, say, a SSD? I am thinking of backing up this RAID10 on a 2x8TB device-managed SMR RAID1 and I want to minimize random write operations (noatime al.). I will start a new thread for that maybe but first, is there something substantial I can read about btrfs+SMR? Or should I avoid SMR+btfs ? For maintenance, I would suggest running a scrub regularly, to check for various forms of bitrot. Typical frequencies for a scrub are once a week or once a month -- opinions vary (as do runtimes). Yes. I cronned it weekly for now. Takes about 5 hours. Is it automatically corrected on RAID10 since a copy of it exist within the filesystem ? What happens for RAID0 ? For raid10 (and the raid1 I use), yes, it's corrected, from the other existing copy, assuming it's good, tho if there are metadata checksum errors, there may be corresponding unverified checksums as well, where the verification couldn't be done because the metadata containing the checksums was bad. Thus, if there are errors found and corrected, and you see unverified errors as well, rerun the scrub, so the newly corrected metadata can now be used to verify the previously unverified errors. ok then, rule of the thumb re-run the scrub on “unverified checksum error(s)”. I have yet to see checksum errors yet but will keep it in mind.. I'm presently getting a lot of experience with this as one of the ssds in my raid1 is gradually failing and rewriting sectors. Generally what happens is that the ssd will take too long, triggering a SATA reset (30 second timeout), and btrfs will call that an error. The scrub then rewrites the bad copy on the unreliable device with the good copy from the more reliable device, with the write triggering a sector relocation on the bad device. The newly written copy then checks out good, but if it was metadata, it very likely contained checksums for several other blocks, which couldn't be verified because the block containing their checksums was itself bad. Typically I'll see dozens to a couple hundred unverified errors for every bad metadata block rewritten in this way. Rerunning the scrub then either verifies or fixes the previously unverified blocks, tho sometimes one of those in turn ends up bad and if it's a metadata block, I may end up rerunning the scrub another time or two, until everything checks out. FWIW, on the bad device, smartctl -A reports (excerpted): ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0032 098 098 036Old_age Always - 259 182 Erase_Fail_Count_Total 0x0032 100 100 000Old_age Always - 132 While on the paired good device: 5 Reallocated_Sector_Ct 0x0032 253 253 036Old_age Always - 0 182 Erase_Fail_Count_Total 0x0032 253 253 000Old_age Always - 0 Meanwhile, smartctl -H has already warned once that the device is failing, tho it went back to passing status again, but as of now it's saying failing, again. The attribute that actually registers as failing, again from the bad device, followed by the good, is: 1 Raw_Read_Error_Rate 0x000f 001 001 006Pre-fail Always FAILING_NOW 3081 1 Raw_Read_Error_Rate 0x000f 160 159 006Pre-fail Always - 41 When it's not actually reporting failing, the FAILING_NOW status is replaced with IN_THE_PAST. 250 Read_Error_Retry_Rate is the other attribute of interest, with values of 100 current and worst for both devices, threshold 0, but a raw value of 2488 for the good device and over 17,000,000 for the failing device. But with the cooked value never moving from 100 and with no real guidance on how to interpret the raw values, while
RAID10 Balancing Request for Comments and Advices
Hello, I have a Centos 7 machine with the latest EPEL kernel-ml (4.0.5) with a 6-disk 4TB HGST RAID10 btrfs volume. With the following mount options : noatime,compress=zlib,space_cache 0 2 btrfs filesystem df” gives : Data, RAID10: total=7.08TiB, used=7.02TiB Data, single: total=8.00MiB, used=0.00B System, RAID10: total=7.88MiB, used=656.00KiB System, single: total=4.00MiB, used=0.00B Metadata, RAID10: total=9.19GiB, used=7.56GiB Metadata, single: total=8.00MiB, used=0.00B GlobalReserve, single: total=512.00MiB, used=0.00B My first question is this : is it normal to have “single” blocks ? Why not only RAID10? I don’t remember the exact mkfs options I used but I certainly didn’t ask for “single” so this is unexpected. My second question is : what is the best device add / balance sequence to use if I want to add 2 more disks to this RAID10 volume? Also is a balance necessary at all since I’m adding a pair? My third question is: given that this file system is an offline backup for another RAID0 volume with SMB sharing, what is the best maintenance schedule as long as it is offline? For now, I only have a weekly cron scrub now, but I think that the priority is to have it balanced after a send-receive or rsync to optimize storage space availability (over performance). Is there a “light” balancing method recommended in this case? My fourth question, still within the same context: are there best practices when using smartctl for periodically testing (long test, short test) btrfs RAID devices? Thanks! Vincent -- To unsubscribe from this list: send the line unsubscribe linux-btrfs in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: RAID10 Balancing Request for Comments and Advices
On Jun 16, 2015, at 8:25 AM, Hugo Mills h...@carfax.org.uk wrote: On Tue, Jun 16, 2015 at 08:09:17AM -0400, Vincent Olivier wrote: btrfs filesystem df” gives : Data, RAID10: total=7.08TiB, used=7.02TiB Data, single: total=8.00MiB, used=0.00B System, RAID10: total=7.88MiB, used=656.00KiB System, single: total=4.00MiB, used=0.00B Metadata, RAID10: total=9.19GiB, used=7.56GiB Metadata, single: total=8.00MiB, used=0.00B GlobalReserve, single: total=512.00MiB, used=0.00B My first question is this : is it normal to have “single” blocks ? Why not only RAID10? I don’t remember the exact mkfs options I used but I certainly didn’t ask for “single” so this is unexpected. Yes. It's an artefact of the way that mkfs works. If you run a balance on those chunks, they'll go away. (btrfs balance start -dusage=0 -musage=0 /mountpoint) Thanks! I did and it did go away, except for the GlobalReserve, single: total=512.00MiB, used=0.00B”. But I suppose this is a permanent fixture, right? My second question is : what is the best device add / balance sequence to use if I want to add 2 more disks to this RAID10 volume? Also is a balance necessary at all since I’m adding a pair? Add both devices first, then balance. For a RAID-1 filesystem, adding two devices wouldn't need a balance to get full usage out of the new devices. However, you've got RAID-10, so the most you'd be able to get on the FS without a balance is four times the remaining space on one of the existing disks. The chunk allocator for RAID-10 will allocate as many chunks as it can in an even number across all the devices, omitting the device with the smallest free space if there's an odd number of devices. It must have space on at least four devices, so adding two devices means that it'll have to have free space on at least two of the existing ones (and will try to use all of them). So yes, unless you're adding four devices, a rebalance is required here. It is perfectly clear and logical that 1+0 works on four devices at a time. My third question is: given that this file system is an offline backup for another RAID0 volume with SMB sharing, what is the best maintenance schedule as long as it is offline? For now, I only have a weekly cron scrub now, but I think that the priority is to have it balanced after a send-receive or rsync to optimize storage space availability (over performance). Is there a “light” balancing method recommended in this case? You don't need to balance after send/receive or rsync. If you find that you have lots of data space allocated but not used (the first line in btrfs fi df, above), *and* metadata close to usage (within, say, 700 MiB), *and* no unallocated space (btrfs fi show), then it's worth running a filtered balance with -dlimit=3 or some similar small value to free up some space that the metadata can expand into. Other than that, it's pretty much entirely pointless. Ok thanks. Is there a btrfs-utils way of automating the if less than 1Gb free do balance -dlimit=3” ? For maintenance, I would suggest running a scrub regularly, to check for various forms of bitrot. Typical frequencies for a scrub are once a week or once a month -- opinions vary (as do runtimes). Yes. I cronned it weekly for now. Takes about 5 hours. Is it automatically corrected on RAID10 since a copy of it exist within the filesystem ? What happens for RAID0 ? Thanks! V-- To unsubscribe from this list: send the line unsubscribe linux-btrfs in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: RAID10 Balancing Request for Comments and Advices
On Tue, Jun 16, 2015 at 08:09:17AM -0400, Vincent Olivier wrote: Hello, I have a Centos 7 machine with the latest EPEL kernel-ml (4.0.5) with a 6-disk 4TB HGST RAID10 btrfs volume. With the following mount options : noatime,compress=zlib,space_cache 0 2 btrfs filesystem df” gives : Data, RAID10: total=7.08TiB, used=7.02TiB Data, single: total=8.00MiB, used=0.00B System, RAID10: total=7.88MiB, used=656.00KiB System, single: total=4.00MiB, used=0.00B Metadata, RAID10: total=9.19GiB, used=7.56GiB Metadata, single: total=8.00MiB, used=0.00B GlobalReserve, single: total=512.00MiB, used=0.00B My first question is this : is it normal to have “single” blocks ? Why not only RAID10? I don’t remember the exact mkfs options I used but I certainly didn’t ask for “single” so this is unexpected. Yes. It's an artefact of the way that mkfs works. If you run a balance on those chunks, they'll go away. (btrfs balance start -dusage=0 -musage=0 /mountpoint) My second question is : what is the best device add / balance sequence to use if I want to add 2 more disks to this RAID10 volume? Also is a balance necessary at all since I’m adding a pair? Add both devices first, then balance. For a RAID-1 filesystem, adding two devices wouldn't need a balance to get full usage out of the new devices. However, you've got RAID-10, so the most you'd be able to get on the FS without a balance is four times the remaining space on one of the existing disks. The chunk allocator for RAID-10 will allocate as many chunks as it can in an even number across all the devices, omitting the device with the smallest free space if there's an odd number of devices. It must have space on at least four devices, so adding two devices means that it'll have to have free space on at least two of the existing ones (and will try to use all of them). So yes, unless you're adding four devices, a rebalance is required here. My third question is: given that this file system is an offline backup for another RAID0 volume with SMB sharing, what is the best maintenance schedule as long as it is offline? For now, I only have a weekly cron scrub now, but I think that the priority is to have it balanced after a send-receive or rsync to optimize storage space availability (over performance). Is there a “light” balancing method recommended in this case? You don't need to balance after send/receive or rsync. If you find that you have lots of data space allocated but not used (the first line in btrfs fi df, above), *and* metadata close to usage (within, say, 700 MiB), *and* no unallocated space (btrfs fi show), then it's worth running a filtered balance with -dlimit=3 or some similar small value to free up some space that the metadata can expand into. Other than that, it's pretty much entirely pointless. For maintenance, I would suggest running a scrub regularly, to check for various forms of bitrot. Typical frequencies for a scrub are once a week or once a month -- opinions vary (as do runtimes). My fourth question, still within the same context: are there best practices when using smartctl for periodically testing (long test, short test) btrfs RAID devices? I can't answer that one, I'm afraid. Hugo. -- Hugo Mills | Welcome to Rivendell, Mr Anderson... hugo@... carfax.org.uk | http://carfax.org.uk/ | PGP: E2AB1DE4 |Machinae Supremacy, Hybrid signature.asc Description: Digital signature
Re: RAID10 Balancing Request for Comments and Advices
On Tue, Jun 16, 2015 at 5:58 PM, Duncan 1i5t5.dun...@cox.net wrote: On a current kernel unlike older ones, btrfs actually automates entirely empty chunk reclaim, so this problem doesn't occur anything close to near as often as it used to. However, it's still possible to have mostly but not entirely empty chunks that btrfs won't automatically reclaim. A balance can be used to rewrite and combine these mostly empty chunks, reclaiming the space saved. This is what Hugo was recommending. Yes, as little as a -dusage=5 (data chunks that are 5% or less full) can clear the problem and is very fast, seconds. Possibly a bit longer, many seconds o single digit minutes is -dusage=15. I haven't done a full balance in forever. -- Chris Murphy -- To unsubscribe from this list: send the line unsubscribe linux-btrfs in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: RAID10 Balancing Request for Comments and Advices
Vincent Olivier posted on Tue, 16 Jun 2015 09:34:29 -0400 as excerpted: On Jun 16, 2015, at 8:25 AM, Hugo Mills h...@carfax.org.uk wrote: On Tue, Jun 16, 2015 at 08:09:17AM -0400, Vincent Olivier wrote: My first question is this : is it normal to have “single” blocks ? Why not only RAID10? I don’t remember the exact mkfs options I used but I certainly didn’t ask for “single” so this is unexpected. Yes. It's an artefact of the way that mkfs works. If you run a balance on those chunks, they'll go away. (btrfs balance start -dusage=0 -musage=0 /mountpoint) Thanks! I did and it did go away, except for the GlobalReserve, single: total=512.00MiB, used=0.00B”. But I suppose this is a permanent fixture, right? Yes. GlobalReserve is for short-term btrfs-internal use, reserved for times when btrfs needs to (temporarily) allocate some space in ordered to free space, etc. It's always single, and you'll rarely see anything but 0 used except perhaps in the middle of a balance or something. You don't need to balance after send/receive or rsync. If you find that you have lots of data space allocated but not used (the first line in btrfs fi df, above), *and* metadata close to usage (within, say, 700 MiB), *and* no unallocated space (btrfs fi show), then it's worth running a filtered balance with -dlimit=3 or some similar small value to free up some space that the metadata can expand into. Other than that, it's pretty much entirely pointless. Ok thanks. Is there a btrfs-utils way of automating the if less than 1Gb free do balance -dlimit=3” ? On a current kernel unlike older ones, btrfs actually automates entirely empty chunk reclaim, so this problem doesn't occur anything close to near as often as it used to. However, it's still possible to have mostly but not entirely empty chunks that btrfs won't automatically reclaim. A balance can be used to rewrite and combine these mostly empty chunks, reclaiming the space saved. This is what Hugo was recommending. Mostly- empty chunk rebalance and reclaim is not (yet?) entirely automated, but in most use-cases it's not something you need to worry about that much. You do it if you notice a huge difference in available vs used in btrfs fi df, or if btrfs fi show drops below several gigs available, or if you start getting nospace errors, but otherwise, don't worry about it. Tho of course you could script the check if desired, but then you'd simply script conditional logic to support your own special-case. For maintenance, I would suggest running a scrub regularly, to check for various forms of bitrot. Typical frequencies for a scrub are once a week or once a month -- opinions vary (as do runtimes). Yes. I cronned it weekly for now. Takes about 5 hours. Is it automatically corrected on RAID10 since a copy of it exist within the filesystem ? What happens for RAID0 ? For raid10 (and the raid1 I use), yes, it's corrected, from the other existing copy, assuming it's good, tho if there are metadata checksum errors, there may be corresponding unverified checksums as well, where the verification couldn't be done because the metadata containing the checksums was bad. Thus, if there are errors found and corrected, and you see unverified errors as well, rerun the scrub, so the newly corrected metadata can now be used to verify the previously unverified errors. I'm presently getting a lot of experience with this as one of the ssds in my raid1 is gradually failing and rewriting sectors. Generally what happens is that the ssd will take too long, triggering a SATA reset (30 second timeout), and btrfs will call that an error. The scrub then rewrites the bad copy on the unreliable device with the good copy from the more reliable device, with the write triggering a sector relocation on the bad device. The newly written copy then checks out good, but if it was metadata, it very likely contained checksums for several other blocks, which couldn't be verified because the block containing their checksums was itself bad. Typically I'll see dozens to a couple hundred unverified errors for every bad metadata block rewritten in this way. Rerunning the scrub then either verifies or fixes the previously unverified blocks, tho sometimes one of those in turn ends up bad and if it's a metadata block, I may end up rerunning the scrub another time or two, until everything checks out. FWIW, on the bad device, smartctl -A reports (excerpted): ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0032 098 098 036Old_age Always - 259 182 Erase_Fail_Count_Total 0x0032 100 100 000Old_age Always - 132 While on the paired good device: 5 Reallocated_Sector_Ct 0x0032 253 253 036Old_age Always - 0 182 Erase_Fail_Count_Total 0x0032 253 253 000Old_age Always
