Re: [PATCH] 498+ days uptime

[Bernhard Heidegger <bheide@hyperwave.com>]

>>>>> ">" == Eric W Biederman <ebiederm@inetnebr.com> writes:

>>>> No.  Major performance problem.

BH> Why?

BH> Imagine an application which has most of the (index) file pages in memory
BH> and many of the pages are dirty. bdflush will flush the pages regularly,
BH> but the pages will get dirty immediately again.
BH> If you can be sure, that the power cannot fail the performance should be
BH> much better without bdflush, because kflushd has to write pages only if
BH> the system is running low on memory...

>> The performance improvement comes when looking for free memory.  In
>> most cases bdflush's slow but steady writing of pages keeps buffers
>> clean.  When the application wants more memory with bdflush in the
>> background unsually the pages it needs will be clean (because the I/O
>> started before the application needed it), so they can just be dropped
>> out of memory.  Relying on kflushd means nothing is written until an
>> application needs the memory and then it must wait until something is
>> written to disk, which is much slower.

>> Further 
>> a) garanteeing no power failure is hard.

Use and UPS and regularly flush/sync the primary data to disk from
the application

>> b) generally there is so much data on the disk you must write it
>>    sometime, because you can't hold it all in memory.

only a question of how much RAM you can put in your PC

>> c) I have trouble imagining a case where a small file would be rewritten
>>    continually.

Not really small, but a database application may use btree based indexes,
where many blocks will get dirty when inserting/deleting data. If you flush
the dirty buffers and the next insertion dirty the same buffer(s) you have
lost performance (Note: the btree based indexes are secondary data; you
can rebuild it from scratch if the system fails)

Bernhard

get my pgp key from a public keyserver (keyID=0x62446355)
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Bernhard Heidegger                                       bheide@hyperwave.com
                  Hyperwave Software Research & Development
                       Schloegelgasse 9/1, A-8010 Graz
Voice: ++43/316/820918-25                             Fax: ++43/316/820918-99
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