max journal size

[Andreas Dilger]

On Feb 15, 2006  18:07 +0100, Jure Pečar wrote:
> Man page of tune2fs says that max journal size is 102,400 filesystem blocks, which translates to ~100MB with 1kb blocks or ~400MB with 4kb block. I wonder - why this limitation exists?

The limit exists to avoid users making the journal too large and consuming
all of their RAM with pinned buffers while the journal is commiting buffers
to the journal and checkpointing them to disk.  Under heavy load it is
possible for jbd to have 3/4*journal_size of lowmem pinned.

> Now that relatively cheap ssd devices exist (gigabyte iRam) that offer up to 4GB of space, it would be extremely useful to use whole capacity of such device for full data journaling.

This limit does not apply when using an external journal.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.