Re: Snapshots

[Pradeep P. <pp...@ee...>]

Umm, interesting. What exactly do you mean by namespace aliasing? An 
example would make things clearer for me.

Marius Eriksen wrote:
> i think that this should be purely a userland administrative thing.
> 
> i.e., all that is needed is the ability to mount a particular
> snapshot.  (and of course, to make said snapshots).
> 
> then, you can have a daemon that manages ~/.snapshot directories. 
> this should be especially doable with the namespace aliasing that
> linux can do.
> 
> marius.
> 
> On 8/23/05, Pradeep Padala <pp...@ee...> wrote:
> 
>>Marius Eriksen wrote:
>>
>>>how about making them mountable filesystems?
>>>
>>>  mount -t lfs -osnapshot="january 12, 13:15" /mnt/old
>>
>>Yep, thought about it, and is probably what I will have as the first
>>version.
>>
>>In NetAPP's WAFL, for each dir .snapshot contains something like
>>hourly.0/ hourly.1/ ... weekly.0/ ...
>>Under these directories you can find snapshots for corresponding
>>directories.
>>
>>This is certainly doable, but requires more careful thought, as we may
>>have to alternate between IFILEs as the user cds back and forth, not
>>trivial. Thoughts ?
>>
>>
>>>On 8/23/05, Pradeep Padala <pp...@ee...> wrote:
>>>
>>>
>>>>Hi,
>>>>  The final piece in the puzzle is in. I have checked in the basic
>>>>framework for doing snapshots. I haven't yet decided on the user
>>>>intreface (may be .snapshots like NetAPP does), but you can access the
>>>>snapshots using lfsread program.
>>>>  I want to explain a little bit about the design.
>>>>  I have toyed around with two different ways of implementing
>>>>snapshots, theoritically actually same. To support any snapshots, one
>>>>has to make a copy of the inode map (in LFS case IFILE) and fs
>>>>modifications are done to the new IFILE only. Also, writes to existing
>>>>blocks (both inode and data) should be re-directed to new blocks (this
>>>>is almost trivial in LFS).
>>>>   Now, how do we make a copy of inode map? We can simply make a new
>>>>IFILE file, copy the contents and start using the new file or We can
>>>>divide the IFILE into snapshots. For example, the snapshot index (snapi)
>>>>can be used to encode within the inode number (say first two bits). So,
>>>>the operations are roughly as follows
>>>>
>>>>1. Implementation using IFILE copy
>>>>   *) the on-disk/in-memory inode structure has snapi.
>>>>   *) user calls LFS_SNAP_CREATE ioctl
>>>>   *) make a copy of IFILE, ++snapi, set the new IFILE inode pointers
>>>>   *) when a file is written its snapi is compared to current snapi, if
>>>>      they differ new block is created.
>>>>   *) to access the snapshot just switch the IFILE. Temporarily
>>>>      switching IFILE back and forth is not that trivial though.
>>>>
>>>>2. Implementation using inode encoding
>>>>   *) For each inode, inode number is derived with a simple function
>>>>      like below
>>>>
>>>>       static inline ino_t INUMBER(struct super_block *sb, ino_t ino)
>>>>       {       __u32 snapi = CURR_SNAPI(sb);
>>>>
>>>>               snapi = snapi << 30;
>>>>               return ino | snapi;
>>>>       }
>>>>
>>>>    *) user calls LFS_SNAP_CREATE ioctl
>>>>    *) make a copy of existing inodes to correponding new inodes. For
>>>>       example inode 1 is copied to 0x4001
>>>>    *) switch the root inode to the new inode.
>>>>    *) here comes the tricky part, the directory entries still contain
>>>>       old inodes. so a readdir always ORs the inode number with current
>>>>       snapi.
>>>>    *) to access the snapshot, just switch to the old root inode
>>>>
>>>>I have actually switched back and forth, but finally decided to stick
>>>>with the first implementation as the dentry contents magic is not
>>>>required. It actually doesn't matter other than at the level of coding
>>>>complexity/clarity. I also kept a local copy of the second
>>>>implementation in case I need it.
>>>>
>>>>Comments are welcome.
>>>>
>>>>lfsprogs/lfssnap.c can be used to make snapshot. lfsprogs/lfsread can be
>>>>used to read the contents of inodes etc. and see the snapshot.
>>>>
>>>>--
>>>>Pradeep Padala
>>>>http://ppadala.blogspot.com