Re: ext4 features (checksums)

[Bill Davidsen <davidsen@tmr.com>]

Avi Kivity wrote:
> Bill Davidsen wrote:
>>
>>
>> > Not syncing unused area is possible, if there was a way for raid resync
>> > to ask the fs what blocks are not in use.  I.e. get the
>> > free block list in disk block order.  Then raid resync could skip 
>> those.
>> >
>> Current RAID code supports having a bitmap of dirty stripes, and can
>> just sync those during recovery. I'm sure Neil could explain it better,
>> but this is available without worrying about fs type. Now. Today.
>>
> 
> This is only when the you reconstruct a disk that was once part of the 
> RAID.  If you are adding a brand new disk, all stripes are dirty.

I will leave Neil to explain this to you, it appears to be a totally 
different case for reconfiguration, but I don't pretend to understand 
the code well enough to clarify it.
> 
> This happens in two scenarios: an unclean RAID shutdown, and when you 
> have a remote mirror which can be disconnected by network problems.
> 
> If the RAID is integrated in the filesystem (or into an object storage 
> system), you can handle the new disk case too.
> 
I'm not sure that building the RAID into the filesystem is ever a good 
idea, it certainly seems likely to either prevent certain RAID devices 
from being used, or make them perform suboptimally. There are times when 
  being able to move a filesystem to a new device is REALLY useful, and 
byte copy is more practical than file by file copy.

-- 
Bill Davidsen <davidsen@tmr.com>
   Obscure bug of 2004: BASH BUFFER OVERFLOW - if bash is being run by a
normal user and is setuid root, with the "vi" line edit mode selected,
and the character set is "big5," an off-by-one errors occurs during
wildcard (glob) expansion.