On 16.11.2017 09:38, Qu Wenruo wrote: > > > On 2017年11月16日 14:54, Nikolay Borisov wrote: >> >> >> On 16.11.2017 04:18, Qu Wenruo wrote: >>> Hi all, >>> >>> [Background] >>> Recently I'm considering the possibility to use checksum from filesystem >>> to enhance device-mapper raid. >>> >>> The idea behind it is quite simple, since most modern filesystems have >>> checksum for their metadata, and even some (btrfs) have checksum for data. >>> >>> And for btrfs RAID1/10 (just ignore the RAID5/6 for now), at read time >>> it can use the checksum to determine which copy is correct so it can >>> return the correct data even one copy get corrupted. >>> >>> [Objective] >>> The final objective is to allow device mapper to do the checksum >>> verification (and repair if possible). >>> >>> If only for verification, it's not much different from current endio >>> hook method used by most of the fs. >>> However if we can move the repair part from filesystem (well, only btrfs >>> supports it yet), it would benefit all fs. >>> >>> [What we have] >>> The nearest infrastructure I found in kernel is bio_integrity_payload. >>> >>> However I found it's bounded to device, as it's designed to support >>> SCSI/SATA integrity protocol. >>> While for such use case, it's more bounded to filesystem, as fs (or >>> higher layer dm device) is the source of integrity data, and device >>> (dm-raid) only do the verification and possible repair. >>> >>> I'm not sure if this is a good idea to reuse or abuse >>> bio_integrity_payload for this purpose. >>> >>> Should we use some new infrastructure or enhance existing >>> bio_integrity_payload? >>> >>> (Or is this a valid idea or just another crazy dream?) >>> >> >> This sounds good in principle, however I think there is one crucial >> point which needs to be considered: >> >> All fs with checksums store those checksums in some specific way, then >> when they fetch data from disk they they also know how to acquire the >> respective checksum. > > Just like integrity payload, we generate READ bio attached with checksum > hook function and checksum data. So how is this checksum data acquired in the first place? > > So for data read, we read checksum first and attach it to data READ bio, > then submit it. > > And for metadata read, in most case the checksum is integrated into > metadata header, like what we did in btrfs. > > In that case we attach empty checksum data to bio, but use metadata > specific function hook to handle it. > >> What you suggest might be doable but it will >> require lower layers (dm) be aware of how to acquire the specific >> checksum for some data. > > In above case, dm only needs to call the verification hook function. > If verification passed, that's good. > If not, try other copy if we have. > > In this case, I don't think dm layer needs any extra interface to > communicate with higher layer. Well that verification function is the interface I meant, you are communicating the checksum out of band essentially (notwithstanding the metadata case, since you said checksum is in the actual metadata header) In the end - which problem are you trying to solve, allow for a generic checksumming layer which filesystems may use if they decide to ? > > Thanks, > Qu > >> I don't think at this point there is such infra >> and frankly I cannot even envision how it will work elegantly. Sure you >> can create a dm-checksum target (which I believe dm-verity is very >> similar to) that stores checksums alongside data but at this point the >> fs is really out of the picture. >> >> >>> Thanks, >>> Qu >>> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> >
