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SubjectRe: RFC - new raid superblock layout for md driver
>The md driver in linux uses a 'superblock' written to all devices in a
>RAID to record the current state and geometry of a RAID and to allow
>the various parts to be re-assembled reliably.
>
>The current superblock layout is sub-optimal. It contains a lot of
>redundancy and wastes space. In 4K it can only fit 27 component
>devices. It has other limitations.

Yes. (In particular, getting all the various counters to agree with each other
is a pain ;-)

Steven raises the valid point that multihost operation isn't currently
possible; I just don't agree with his solution:

- Activating a drive only on one host is already entirely possible.
(can be done by device uuid in initrd for example)
- Activating a RAID device from multiple hosts is still not possible.
(Requires way more sophisticated locking support than we currently have)

However, for none-RAID devices like multipathing I believe that activating a
drive on multiple hosts should be possible; ie, for these it might not be
necessary to scribble to the superblock every time.

(The md patch for 2.4 I sent you already does that; it reconstructs the
available paths fully dynamic on startup (by activating all paths present);
however it still updates the superblock afterwards)

Anyway, on to the format:

>The code in 2.5.lastest has all the superblock handling factored out so
>that defining a new format is very straight forward.
>
>I would like to propose a new layout, and to receive comment on it..
>
>My current design looks like:
> /* constant array information - 128 bytes */
> u32 md_magic
> u32 major_version == 1
> u32 feature_map /* bit map of extra features in superblock */
> u32 set_uuid[4]
> u32 ctime
> u32 level
> u32 layout
> u64 size /* size of component devices, if they are all
> * required to be the same (Raid 1/5 */
> u32 chunksize
> u32 raid_disks
> char name[32]
> u32 pad1[10];

Looks good so far.

> /* constant this-device information - 64 bytes */
> u64 address of superblock in device
> u32 number of this device in array /* constant over reconfigurations
> */
> u32 device_uuid[4]

What is "address of superblock in device" ? Seems redundant, otherwise you
would have been unable to read it, or am missing something?

Special case here might be required for multipathing. (ie, device_uuid == 0)

> u32 pad3[9]
>
> /* array state information - 64 bytes */
> u32 utime

Timestamps (also above, ctime) are always difficult. Time might not be set
correctly at any given time, in particular during early bootup. This field
should only be advisory.

> u32 state /* clean, resync-in-progress */
> u32 sb_csum
> u64 events
> u64 resync-position /* flag in state if this is valid)
> u32 number of devices
> u32 pad2[8]
>
> /* device state information, indexed by 'number of device in array'
> 4 bytes per device */
> for each device:
> u16 position /* in raid array or 0xffff for a spare. */
> u16 state flags /* error detected, in-sync */

u16 != u32; your position flags don't match up. I'd like to be able to take
the "position in the superblock" as a mapping here so it can be found in this
list, or what is the proposed relationship between the two?

>The interpretation of the 'name' field would be up to the user-space
>tools and the system administrator.
>I imagine having something like:
> host:name
>where if "host" isn't the current host name, auto-assembly is not
>tried, and if "host" is the current host name then:

Oh, well. You seem to sort of have Steven's idea here too ;-) In that case,
I'd go with the idea of Steven. Make that field a uuid of the host.



Sincerely,
Lars Marowsky-Brée <lmb@suse.de>

--
Principal Squirrel
SuSE Labs - Research & Development, SuSE Linux AG

"If anything can go wrong, it will." "Chance favors the prepared (mind)."
-- Capt. Edward A. Murphy -- Louis Pasteur
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