Excerpts from liubo's message of 2011-03-16 22:10:09 -0400: > On 03/16/2011 05:06 PM, Amir Goldstein wrote: > > On Wed, Mar 16, 2011 at 1:35 AM, Chris Mason <chris.ma...@oracle.com> wrote: > >> Excerpts from Andreas Dilger's message of 2011-03-15 18:06:49 -0400: > >>> On 2011-03-15, at 2:57 PM, Christoph Hellwig wrote: > >>>> On Tue, Mar 15, 2011 at 04:26:50PM -0400, Chris Mason wrote: > >>>>> #define FS_EXTENT_FL 0x00080000 /* Extents */ > >>>>> #define FS_DIRECTIO_FL 0x00100000 /* Use direct i/o */ > >>>>> +#define FS_NOCOW_FL 0x00800000 /* Do not cow file */ > >>>>> +#define FS_COW_FL 0x01000000 /* Cow file */ > >>>>> #define FS_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > >>>> I'm fine with it. I'll defer the check for conflicts with extN-specific > >>>> flags > >>>> to Ted, though. > >>> Looking at the upstream e2fsprogs I see in that range: > >>> > >>>> #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */ > >>>> #define EXT4_EA_INODE_FL 0x00200000 /* Inode used for large EA > >>>> */ > >>>> #define EXT4_EOFBLOCKS_FL 0x00400000 /* Blocks allocated beyond > >>>> EOF */ > >>>> #define EXT4_SNAPFILE_FL 0x01000000 /* Inode is a snapshot */ > >>>> #define EXT4_SNAPFILE_DELETED_FL 0x04000000 /* Snapshot is being > >>>> deleted */ > >>>> #define EXT4_SNAPFILE_SHRUNK_FL 0x08000000 /* Snapshot shrink has > >>>> completed */ > >>>> #define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */ > >>>> > >>>> #define EXT2_FL_USER_VISIBLE 0x004BDFFF /* User visible flags */ > >>> so there is a conflict with FS_COW_FL and EXT4_SNAPFILE_FL. I don't know > >>> the semantics of those two flags enough to say for sure whether it is > >>> reasonable that they alias to each other, but at first glance "COW" and > >>> "SNAPSHOT" don't seem completely unrelated. > > > > EXT4_SNAPFILE_FL indicates a special system snapshot file, so it has > > no equivalence relation with FS_COW_FL. > > Please use 0x02000000 for FS_COW_FL. > > Fine with that, but it's up to Chris. :)
I'd rather not conflict unless we're critically short on space. > > > > EXT4_SNAPFILE_DELETED_FL is a persistent state of a snapshot file, > > which is no longer > > available as a mountable device, but cannot be unlinked because it > > holds changed data sets > > needed by older snapshots. > > > > EXT4_SNAPFILE_SHRUNK_FL is a persistent state of a (deleted) snapshot > > file, which has > > undergone a "shrink" process to free all change sets not needed by > > older snapshots. > > The persistence of the flag is needed to avoid tedious shrinking when > > it is not needed. > > > > > >> In the btrfs case FS_COW_FL means to do COW even when there are no > >> snapshots. FS_NOCOW_FL means to do cow only when there are snapshots. > >> > > > > I am interested in FS_NOCOW_FL as well, but for my implementation it would > > mean > > do not do COW on rewrites even when there are snapshots, so a user can > > create a pre-allocated > > "island of blocks", which are pinned to a physical location, for raw > > VM image for example. I'm not sure how the island of blocks idea can work with snapshots? Wouldn't the snapshot corrupt if anything in the island were changed? -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
