Garrett: ext4, application expectations and power management [LWN.net]

Garrett: ext4, application expectations and power management

Posted Mar 16, 2009 9:58 UTC (Mon) by mjthayer (guest, #39183)
In reply to: Garrett: ext4, application expectations and power management by drag
Parent article: Garrett: ext4, application expectations and power management

I have asked this a couple of times but not yet got a good answer. I presume that the kernel knows what has been written back and what not. Can't it optionally keep its own log - either in a file on the filesystem or in pre-allocated blocks on a swap device - where it writes details of any transaction which the target filesystem won't write back within a certain maximum timeframe. When the filesystem does do the writeback the transaction can be purged from the log. This could be enabled or disabled for the entire system, regardless of what filesystems are in use, and would not require Ted to add code he doesn't like.

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Garrett: ext4, application expectations and power management

Posted Mar 16, 2009 10:52 UTC (Mon) by MathFox (guest, #6104) [Link] (2 responses)

Michael, Yes, the kernel could do it, but such a log would have to be written to disk... But then it would be more efficient to directly write that log directly to the file system.
You'll create similar issues wrt. performance and commit intervals with a kernel-based log, but with the added overhead of writing data twice.

Garrett: ext4, application expectations and power management

Posted Mar 16, 2009 10:54 UTC (Mon) by mjthayer (guest, #39183) [Link] (1 responses)

Would that apply even if the blocks for the log were reserved in advance and their location known to the kernel?

Garrett: ext4, application expectations and power management

Posted Mar 16, 2009 10:55 UTC (Mon) by mjthayer (guest, #39183) [Link]

I will answer my own question - presumably yes, because the kernel can't assume that the filesystem does a simple block to disk mapping.