Date:	Mon, 24 Mar 2014 19:48:04 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Lukáš Czerner <lczerner@...hat.com>
Cc:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: Proposal draft for data checksumming for ext4

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@...hat.com> 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@...cle.com>
>> To: Lukáš Czerner <lczerner@...hat.com>
>> Cc: linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
>> 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
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