Some quick thoughts in this: - checksum blocks should cover all non-static blocks in the group, don't need separate checksums for itable, bitmap, and descriptors - if it is complex to skip static blocks with their own checksums, just leave those blocks empty (zero checksum). - address in group descriptor should be relative to other group metadata, for example the block bitmap, so it works with/without flex_bg and it is clear that "0" is an invalid checksum block array address Cheers, Andreas > On Mar 23, 2014, at 19:59, Lukáš Czerner <lczerner@xxxxxxxxxx> wrote: > >> On Thu, 20 Mar 2014, Darrick J. Wong wrote: >> >> Date: Thu, 20 Mar 2014 10:59:50 -0700 >> From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> >> To: Lukáš Czerner <lczerner@xxxxxxxxxx> >> Cc: linux-ext4@xxxxxxxxxxxxxxx, Theodore Ts'o <tytso@xxxxxxx> >> Subject: Re: Proposal draft for data checksumming for ext4 >> >>> On Thu, Mar 20, 2014 at 05:40:06PM +0100, Lukáš Czerner wrote: >>> Hi all, >>> >>> I've started thinking about implementing data checksumming for ext4 file >>> system. This is not meant to be a formal proposal or a definitive design >>> description since I am not that far yet, but just a few ideas to start >>> the discussion and trying to figure out what the best design for data >>> checksumming in ext4 might be. >>> >>> >>> >>> Data checksumming for ext4 >>> Version 0.1 >>> March 20, 2014 >>> >>> >>> Goal >>> ==== >>> >>> The goal is to implement data checksumming for ext4 file system in order >>> to improve data integrity and increase protection against silent data >>> corruption while maintaining reasonable performance and usability of the >>> file system. >>> >>> While data checksums can be certainly used in different ways, for example >>> data deduplication this proposal is very much focused on data integrity. >>> >>> >>> Checksum function >>> ================= >>> >>> By default I plan to use crc32c checksum, but I do not see a reason why not >>> not to be able to support different checksum function. Also by default the >>> checksum size should be 32 bits, but the plan is to make the format >>> flexible enough to be able to support different checksum sizes. >> >> <nod> Were you thinking of allowing the use of different functions for data and >> metadata checksums? > > Hi Darrick, > > I have not, but I think that this would be very easy to do if we can > agree that it's good to have. > > >> >>> Checksumming and Validating >>> =========================== >>> >>> On write checksums on the data blocks need to be computed right before its >>> bio is submitted and written out as metadata to its position (see bellow) >>> after the bio completes (similarly as we do unwritten extent conversion >>> today). >>> >>> Similarly on read checksums needs to be computed after the bio completes >>> and compared with the stored values to verify that the data is intact. >>> >>> All of this should be done using workqueues (Concurrency Managed >>> Workqueues) so we do not block the other operations and to spread the >>> checksum computation and comparison across CPUs. One wq for reads and one >>> for writes. Specific setup of the wq such as priority, or concurrency limits >>> should be decided later based on the performance evaluation. >>> >>> While we already have ext4 infrastructure to submit bios in >>> fs/ext4/page-io.c where the entry point is ext4_bio_write_page() we would >>> need the same for reads to be able to provide ext4 specific hooks for >>> io completion. >>> >>> >>> Where to store the checksums >>> ============================ >>> >>> While the problems above are pretty straightforward when it comes to the >>> design, actually storing and retrieving the data checksums from to/from >>> the ext4 format requires much more thought to be efficient enough and play >>> nicely with the overall ext4 design while trying not to be too intrusive. >>> >>> I came up with several ideas about where to store and how to access data >>> checksums. While some of the ideas might not be the most viable options, >>> it's still interesting to think about the advantages and disadvantages of >>> each particular solution. >>> >>> a) Static layout >>> ---------------- >>> >>> This scheme fits perfectly into the ext4 design. Checksum blocks >>> would be preallocated the same way as we do with inode tables for example. >>> Each block group should have it's own contiguous region of checksum blocks >>> to be able to store checksums for bocks from entire block group it belongs >>> to. Each checksum block would contain header including checksum of the >>> checksum block. >>> >>> We still have unused 4 Bytes in the ext4_group_desc structure, so storing >>> a block number for the checksum table should not be a problem. >> >> What if you have a 64bit filesystem? Do you have some strategy in mind to work >> around that? What about the snapshot exclusion bitmap field? Afaict that >> never went in, so perhaps that field could be reused? > > Yes we can use the exclusion bitmap field. I think that would not be > a problem. We could also use addressing from the start of the block > group and keep the checksum table in the block group. > >> >>> Finding a checksum location of each block in the block group should be done >>> in O(1) time, which is very good. Other advantage is a locality with the >>> data blocks in question since both resides in the same block group. >>> >>> Big disadvantage is the fact that this solution is not very flexibile which >>> comes from the fact that the location of "checksum table" is statically >>> located at a precise position in the file system at mkfs time. >> >> Having a big dumb block of checksums would be easier to prefetch from disk for >> fsck and kernel driver, rather than having to dig through some tree structure. >> (More on that below) > > I agree, it is also much more robust solution than having a tree. > >> >>> There are also other problems we should be concerned with. Ext4 file system >>> does have support for metadata checksumming so all the metadata does have >>> its own checksum. While we can avoid unnecessarily checksuming inodes, group >>> descriptors and basicall all statically positioned metadata, we still have >>> dynamically allocated metadata blocks such as extent blocks. These block >>> do not have to be checksummed but we would still have space reserved in the >>> checksum table. >> >> Don't forget directory blocks--they (should) have checksums too, so you can >> skip those. >> >> I wonder, could we use this table to store backrefs too? It would make the >> table considerably larger, but then we could (potentially) reconstruct broken >> extent trees. > > Definitely, that is one thing I did not discussed here, but I'd like > to have the checksum blocks self descriptive so we can alway know > where it belongs and who is the owner. So yes, having a backrefs is > really good idea. > >> >>> I think that we should be able to make this feature without introducing any >>> incompatibility, but it would make more sense to make it RO compatible only >>> so we can preserve the checksums. But that's up to the implementation. >> >> I think you'd have to have it be rocompat, otherwise you could write data with >> an old kernel and a new kernel would freak out. > > Yes, I think that we could make it not freak out, but we would loose > the checksums, so for that I think that having this rocompat will > probably make more sense. > > Thanks! > -Lukas > >> >>> b) Special inode >>> ---------------- >>> >>> This is very "lazy" solution and should not be difficult to implement. The >>> idea is to have a special inode which would store the checksum blocks in >>> it's own data blocks. >>> >>> The big disadvantage is that we would have to walk the extent tree twice for >>> each read, or write. There is not much to say about this solution other than >>> again we can make this feature without introducing any incompatibility, but >>> it would probably make more sense to make it RO compatible to preserve the >>> checksums. >>> >>> c) Per inode checksum b-tree >>> ---------------------------- >>> >>> See d) >>> >>> d) Per block group checksum b-tree >>> ---------------------------------- >>> >>> Those two schemes are very similar in that both would store checksum in a >>> b-tree with a block number (we could use logical block number in per inode >>> tree) as a key. Obviously finding a checksum would be in logarithmic time, >>> while the size of the tree would be possibly much bigger in the per-inode >>> case. In per block group case we will have much smaller boundary of >>> number of checksum blocks stored. >>> >>> This and the fact that we would have to have at least one checksum block >>> per inode (which would be wasteful in the case of small files) is making per >>> block group solution much more viable. However the major disadvantage of >>> per block group solution is that the checksum tree would create a source of >>> contention when reading/writing from/to a different inodes in the same block >>> group. This might be mitigated by having a worker thread per a range of block >>> groups - but it might still be a bottleneck. >>> >>> Again we still have 4 Bytes in ext4_group_desc to store the pointer to the >>> root of the tree. While the ext4_inode structure have 4Bytes of >>> i_obso_faddr but that's not enough. So we would have to figure out where to >>> store it - we could possibly abuse i_block to store it along with the extent >>> nodes. >> >> I think(?) your purpose in using either a special inode or a btree to store the >> checksums is to avoid wasting checksum blocks on things that are already >> checksummed? I'm not sure that we'd save enough space to justify the extra >> processing. >> >> --D >> >>> File system scrub >>> ================= >>> >>> While this is certainly a feature which we want to have in both userspace >>> e2fsprogs and kernel I do not have any design notes at this stage. >>> >>> >>> >>> >>> I am sure that there are other possibilities and variants of those design >>> ideas, but I think that this should be enough to have a discussion started. >>> As I is not I think that the most viable option is d) that is, per block >>> group checksum tree, which gives us enough flexibility while not being too >>> complex solution. >>> >>> I'll try to update this description as it will be getting more concrete >>> structure and I hope that we will have some productive discussion about >>> this at LSF. >>> >>> Thanks! >>> -Lukas >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
