NVCache for journal

Posted Mar 23, 2007 3:56 UTC (Fri) by sweikart (guest, #4276)
Parent article: The 2007 Linux Storage and File Systems Workshop

> Also, a file system can use the NVCache to store its journal ...

This seems like a great approach. If the NVCache could appear as
a block device that could be partitioned, filesystems that use jbd
(like ext3) could use it right away (a smaller NVCache partition for
read-mostly filesystems like /usr, a larger NVCache partition for
write-mostly filesystems like /var).

I assume journals are treated as ring buffers for writing, which
is the right access pattern for prolonging the life of flash.

-scott

NVCache for journal

Posted Mar 25, 2007 1:48 UTC (Sun) by dlang (guest, #313) [Link] (1 responses)

useing flash for a journal would have some headaches (potentially fixable)

first, every write on the system would have to go through the flash, so longest life access pattern or not, it may still have problems.

second, when the write is complete the system needs to go back to the hjournal (flash) and mark it as being completed

third, the chunks of data going into the journal are of many different sizes (yes, in the end it all gets down to writing fixed size disk blocks, but is that the most efficant method to record an atime update in a journal? probably not)

good drivers and layouts that are flash aware may be abel to address these, if the API gives them sufficiant control (for example, since flash can be chagned from 0 to 1 without eraseing, make sure the 'entry commited flag is 1 when complete and you can just flip that bit, in theory)

NVCache for journal

Posted Mar 25, 2007 2:20 UTC (Sun) by sweikart (guest, #4276) [Link]

> first, every write on the system would have to go through the
> flash, so longest life access pattern or not, it may still have
> problems.

Actually, for most of us journal users, only the metadata writes
go into the journal.

> ... is that the most efficient method to record an atime update
> in a journal?

You raise a good point. As a system administrator, I don't mount
with noatime, because atime is too useful. But, atime is not so
critical that I need it to be journaled; so, I'd love to mount
nojournaledatime.

-scott