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Re: performance gain in data journalling mode



On Tue, Nov 25, 2003 at 01:57:33PM +0000, mb/ext3 dcs qmul ac uk wrote:
> On Nov 22 Wen-han Goh wrote:
> 
> >If I understand correctly, full journalling mode gives better 
> >performance for applications that do a lot of updates in O_SYNC. Could 
> >you please explain how this is possible? Doesn't full data journalling 
> >do twice as many writes as meta data journalling?
> 
> Yes it does; however, once the data is in the journal the O_SYNC caller 
> can continue. With other modes you have to wait until the journal is 
> committed, and all the extra seeks that may ensue...

More to the point, writes to the journal don't reqiure any seeks.  So
once the data has been written to the journal, you don't have to wait
for the data to be written to their final locations on disk.  This can
result in faster times than not doing any journalling at all, because
of the ability to reduce seeks.  This is especially true if the
journal is located on a separate block device, either on a separate
disk spingle, or especially if it is a battery-backed up NVRAM
device....

						- Ted




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