On 2009-12-07, at 14:46, Iavor Stoev wrote:
I wonder if the Ext3's MAX journal size of 102,400 file system blocks
has been increased in Ext4.
I'm using 10TB 4k block Ext3 file system with external journal on
Gigabyte I-Ram drive and I'm planning a migration to Ext4 system.
And I wonder if I can increase the journal size over 400MB.
Well, even with ext3 the maximum journal size was only for internal
journals. It was always possible to have larger external journal
devices.
With ext4, the maximum journal size WAS increased, though this is in
fact a mke2fs/tune2fs limit so it is also increased for new ext3
filesystems.
Note that with large journals you are also consuming an equal amount
of RAM as the size of the journal, so don't make it crazy big. Having
a journal on SSD is only really noticable for sync-happy workloads.
It isn't noticably better than using a regular disk for the external
journal if you aren't doing a lot of syncs (e.g. NFS or email).
I've thought in the past that it might be an interesting hack to use a
huge journal device (say 32GB) with data journaling, and then have the
JBD layer get the data blocks from the journal for checkpointing to
the filesystem instead of keeping the buffers pinned in RAM. That
would would allow blazing metadata workloads, zero seeking, and then
checkpointing in bulk to the filesystem. ... but unfortunately not
something I have time to test out.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
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