EXTENT BOUNDARIES

Hello,

We are looking into running XFS on a 3TB FLASH MEMORY MODULE. We have a question regarding the extent boundaries.
See the attached PowerPoint drawing, xfs.ppt We are running Linux.
Our media is 3 million contiguous 4KB blocks. We would like to define an extent size of 1MB and this tracks the erasure block size
of the flash memory, and that greatly improves perfomance. We are trying to understand where XFS places the extent boundaries with reference to the contiguous block sequence. Is this deterministic as indicated in the drawing ? That is, are the extent boundaries on 256 block boundaries.

Any help would be greatly appreciated.

Les Oxley
Ampex Corporation
Redwood City
California.

neat, you need to send me some of those

does it wear level and do other clever stuff? most conventional flash storage devices don't last long with journalling filesystems...

you're asking linux people to read a powerpoint file? :-)

you might have better luck with something else

That's not 3TB, surely it's < 12GB?

i didn't open the powerpoint file, so i migth not be answering this very well...

extent boundaries do not have to be on any boundary

you can tweak mkfs.xfs to affect where AGs start, is that of value to you?

now, there are also rt volumes which can have larger extents so those might be of value to you, but you would still need somewhere for the metadata

extents by definition land on filesystem block boundaries, and can in general be any number of filesystem blocks, starting & ending most anywhere on the block device.

If you wish to always allocate in 1m chunks, you might consider using the xfs realtime subvolume, see the extsize description in the mkfs.xfs man page. I'm not sure how much buffered IO to the realtime subvol has been tested; pretty sure it works at this point, though the sgi guys will correct me if I'm wrong... it's not exactly the normal mode of operation.

Using the realtime subvol, however, all your file -metadata- will still be allocated on the main data volume, in much smaller pieces.

-Eric

If you don't need to use 100% of the space for your data, you can give XFS a hint to align on a stripe unit if it's applicable.

If you allocate 1MB chunks at a time (either via a write or prealloc) with an filesystem with sunit=1MB (2048 sectors if using 512 bytes sectors for mkfs.xfs command), it will align to the stripe unit where there is space available.

Once the aligned space is full, it will allocate in the remaining space. Metadata such as inodes, directories, etc will not be nicely aligned.

If your total write or prealloc is smaller than 512KB it will not nicely align, but find suitable space.

I would recommending some experimentation to see if either of the above ideas are suitable for your purpose.

For the sunit idea and 512 byte sector size, the following mkfs command should work:

# mkfs.xfs -b 4096 -d sunit=2048,swidth=4096 <device>

To see it in action using dd:

# dd if=/dev/zero of=<file> bs=1048576
# xfs_bmap -v <file>

You should see the block range aligned to 2048 sectors.