Date:	Tue, 07 Jun 2011 11:26:43 -0400
From:	Josef Bacik <josef@...hat.com>
To:	amir73il@...rs.sourceforge.net
CC:	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH RFC 00/30] Ext4 snapshots - core patches

On 05/09/2011 12:41 PM, amir73il@...rs.sourceforge.net wrote:
> The following patch series includes all the changes to core ext4 files,
> which are needed for snapshots support. It adds some ~2K lines of code,
> which will never be executed unless the following 2 conditions apply:
> 1. ext4 is built with CONFIG_EXT4_FS_SNAPSHOT
> 2. HAS_SNAPSHOT and EXCLUDE_BITMAP features are set by mke2fs/tune2fs
> 
> The remaining ~5K lines of code, added in new snapshot* files, were omitted
> from this series to simplify the review and becasue they are not needed
> when building ext4 without CONFIG_EXT4_FS_SNAPSHOT.
> the full patches will be posted soon after I recieve some comments.
> 
> Ted has concluded my ext4 snapshots talk on LPC 2010 with the statement that
> as long as the snapshot patches don't break anything when snapshot support
> is disabled, he will pull them, so the main goal when reviewing this series
> should be to prove that it is safe to pull the patches.
> 
> REVIEWING
> ---------
> To make it easy for reviewers, I will provide some pointers:
> - EXT4_SNAPSHOTS(sb) is defined to (0) (in snapshot.h) when ext4 is built
>   without snapshots support.
> - EXT4_SNAPSHOTS(sb) is defined to test the HAS_SNAPSHOT feature when ext4
>   is built with snapshots support.
> - All the ext4_snapshot_XXX function added by the patches, are defined to
>   NOP macros in snapshot.h when ext4 is built without snapshots support.
> - Various flags defined by the patches (like EXT4_MB_HINT_COWING) will never
>   get set if EXT4_SNAPSHOTS(sb) is false, so testing them will also be false.
> 
> MERGING
> -------
> These patches are based on Ted's current master branch + alloc_semp removal
> patches. Although the alloc_semp removal is an independent (and in my eyes
> a good) change, it is also required by snapshot patches, to avoid circular
> locking dependency during COW allocations.
> 
> Merging with Allison's punch holes patches should be straight forward, since
> the hard part, namely Yongqiang's split extent refactoring patches, was
> already merged by Ted.
> 
> Merging with Ted's big alloc patches is going to be a bit more challenging,
> since big alloc patches make a lot of renaming and refactoring. However,
> since has_snapshots and big_alloc features will never work together,
> at least testing the code together is not a big concern.
> 
> TESTING
> -------
> Apart from the extensive testing for the snapshots feature functionality, we
> also ran xfstests with snapshots and while taking a snapshot every 1 minute.
> More importantly, we ran xfstests with snapshots support disabled in compile
> time and with snapshot support enabled but without has_snapshot feature.
> These xfstests were run with blocksize 1K and 4K and on X86 and X86_64.
> The 1K blocksize tests are important for the alloc_semp removal patches.
> No problems were found apart from one (test 225 hung), which is already
> existing in master branch.
> 
> CREDITS
> -------
> The snapshots patches originate in my implementation of the Next3 filesystem
> for CTERA networks.
> The porting of the Next3 snapshot patches to ext4 patches is attributed to
> Aditya Dani, Shardul Mangade, Piyush Nimbalkar and Harshad Shirwadkar from
> the Pune Institute of Computer Technology (PICT).
> The implementation of extents move-on-write, delayed move-on-write and much
> of the cleanup work on these patches was carried out by Yongqiang Yang from
> the Institute of Computing Technology, Chinese Academy of Sciences.
> 

I probably should have brought this up before, but why put all this
effort into shoehorning in such a big an invasive feature to ext4 when
btrfs does this all already?  Why not put your efforts into helping
btrfs become stable and ready and then use that, instead of having to
come up with a bunch of hacks to get around the myriad of weird feature
combinations you can get with ext4?

The wonderful thing about ext4 is its a nice basic fs.  If we're going
to start doing lots of crazy things, why not do them to the fs that
isn't yet in wide use and can afford to have crazy things done to it
without screwing a bunch of users who already depend on ext4's
stability?  Thanks,

Josef
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