[stgt] [PATCH 0/2] RFC - Double-linked list backing store.

[ronnie sahlberg]

On Thu, Aug 21, 2008 at 7:38 PM, Mark Harvey <markh794 at gmail.com> wrote:
> FUJITA Tomonori wrote:
>>
>> On Mon, 18 Aug 2008 15:47:00 +1000
>> Mark Harvey <markh794 at gmail.com> wrote:
>>
>>> An alternative backing store primarily for SSC device which uses a
>>> meta-data header for each block of data.
>>>
>>> Embedded with the metadata are forward & back pointers to next & previous
>>> headers forming a double-linked list of headers.
>>
>> I think that everyone agrees on having the separate metadata. When we
>> discussed this last time, I thought that we use extended attributes to
>> store metadata. Looks like that your code uses the first bytes of a
>> file to store the metadata. Using extended attributes doesn't work?
>
> To be honest, I haven't looked at the extended attribute method. How much
> data can be saved within the extended attributes ?

That differs from filesystem to filesystem.
I think that assuming/requiring more than 8-16kbyte total for extended
attributes for a file  will limit it to a  very small subset of all
filesystems where it can be used.

>
> To support variable block, metadata needs to be kept for each block.
>

Since this will be a structure of a handful of bytes for each of
possibly bilions of blocks   extended attributes might be suboptimal
for storing this partucular metadata.

Due to the massive amount of metadata required for the
"per-block-metadata" you probably want a non-extended attributes
method for that.   separate file?
For all other types of metadata, which may be shared with MMC, which
is also a very small amount of metadata, like media-serial-number,
barcode, mam-data etc    for those i think extended attributes are
best.
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