From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Darrick J. Wong" Subject: Re: [PATCH v2.2 00/23] ext4: Add metadata checksumming Date: Tue, 13 Dec 2011 17:10:28 -0800 Message-ID: <20111214011028.GA8233@tux1.beaverton.ibm.com> References: <20111214004613.28243.54122.stgit@elm3c44.beaverton.ibm.com> Reply-To: djwong@us.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Sunil Mushran , Martin K Petersen , Greg Freemyer , Amir Goldstein , linux-kernel , Andi Kleen , Mingming Cao , Joel Becker , linux-fsdevel , linux-ext4@vger.kernel.org, Coly Li To: Andreas Dilger , Theodore Tso Return-path: Content-Disposition: inline In-Reply-To: <20111214004613.28243.54122.stgit@elm3c44.beaverton.ibm.com> Sender: linux-ext4-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Tue, Dec 13, 2011 at 04:46:14PM -0800, Darrick J. Wong wrote: > Hi all, > > This patchset adds crc32c checksums to most of the ext4 metadata objects. A > full design document is on the ext4 wiki[1] but I will summarize that document here. > > As much as we wish our storage hardware was totally reliable, it is still > quite possible for data to be corrupted on disk, corrupted during transfer over > a wire, or written to the wrong places. To protect against this sort of > non-hostile corruption, it is desirable to store checksums of metadata objects > on the filesystem to prevent broken metadata from shredding the filesystem. > > The crc32c polynomial was chosen for its improved error detection capabilities > over crc32 and crc16, and because of its hardware acceleration on current and > upcoming Intel and Sparc chips. > > Each type of metadata object has been retrofitted to store a checksum as follows: > > - The superblock stores a crc32c of itself. > - Each inode stores crc32c(fs_uuid + inode_num + inode_gen + inode + > slack_space_after_inode) > - Block and inode bitmaps each get their own crc32c(fs_uuid + group_num + > bitmap), stored in the block group descriptor. > - Each extent tree block stores a crc32c(fs_uuid + inode_num + inode_gen + > extent_entries) in unused space at the end of the block. > - Each directory leaf block has an unused-looking directory entry big enough to > store a crc32c(fs_uuid + inode_num + inode_gen + block) at the end of the > block. > - Each directory htree block is shortened to contain a crc32c(fs_uuid + > inode_num + inode_gen + block) at the end of the block. > - Extended attribute blocks store crc32c(fs_uuid + id + ea_block) in the > header, where id is, depending on the refcount, either the inode_num and > inode_gen; or the block number. > - MMP blocks store crc32c(fs_uuid + mmpblock) at the end of the MMP block. > - Block groups can now use crc32c instead of crc16. > - The journal now has a v2 checksum feature flag. > - crc32c(j_uuid + block) checksums have been inserted into descriptor blocks, > commit blocks, revoke blocks, and the journal superblock. > - Each block tag in a descriptor block has a checksum of the related data block. > > The patchset for e2fsprogs will be sent under separate cover only to linux-ext4 > as it is quite lengthy (~48 patches). > > As far as performance impact goes, I see nearly no change with a standard mail > server ffsb simulation. On a test that involves only file creation and > deletion and extent tree modifications, I see a drop of about 50 percent with > the current kernel crc32c implementation; this improves to a drop of about 20 > percent with the enclosed crc32c implementation. However, given that metadata > is usually a small fraction of total IO, it doesn't seem like the cost of > enabling this feature is unreasonable. > > There are of course unresolved issues: > > - I haven't fixed it up to checksum the exclude bitmap yet. I'll probably > submit that as an add-on to the snapshot patchset. > > - Using the journal commit hooks to delay crc32c calculation until dirty > buffers are actually being written to disk. > > - Interaction with online resize code. Yongqiang seems to be in the process of > rewriting this not to use custom metadata block write functions, but I haven't > looked at it very closely yet. > > Please have a look at the design document and patches, and please feel free to > suggest any changes. > > v2: Checksum the MMP block, store the checksum type in the superblock, include > the inode generation in file checksums, and finally solve the problem of limited > space in block groups by splitting the checksum into two halves. > > v2.1: Checksum the reserved parts of the htree tail structure. Fix some flag > handling bugs with the mb cache init routine wherein bitmaps could fail to be > checksummed at read time. > > v2.2: Reincorporate the FS UUID in the bitmap checksum calcuations. Move all > disk layout changes to the front and the feature flag enablement to the end of > the patch set. Fail journal recovery if revoke block fails checksum. > > This patchset has been tested on 3.2.0-rc5 on x64, i386, ppc64, and ppc32. The > patches seems to work fine on all four platforms. OH COME ON!!!! stgit helpfully changed the From lines, screwing everything up. Awesome. I apologize, I wasn't expecting stgit to change the From: lines when I migrated the disk format changes to the front of the set. I don't really want to respam the list just to fix this one little thing. If people want more code changes I'll gladly make them and re-send. If not, then Ted, I can just send you the whole mess as a big mbox file, or post them on a webserver somewhere, with correct attribution. --D > > --D > > [1] https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums > > -- > To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >