Re: [Firebird-devel] forced writes, raw devices and direct IO

[Pavel C. <pc...@ib...>]

Alex Peshkov wrote:
> After fixing http://tracker.firebirdsql.org/browse/CORE-1476 I've made some 
> measurements of database grow time combining different settings of 
> MaxFileSystemCache (set it ot zero to always avoid kernel cache use) and 
> locating database on normal FS or raw device. The results are:
> 
> Location        DIRECT_IO=on      DIRECT_IO=off
> ext3fs		2m 13s            0m 27s
> raw device	0m 34s            0m 20s
> 
> You may see that it looks like now I know an answer to question 'What for do 
> we need that raw devices?'. We need it to accelerate operation in forced 
> write mode. Specially of we need DIRECT_IO to save memory for cache.

While this information is very interesting, I'm not sure whether 
performance with FW ON is the right reason to use raw device. Comparing 
ext3 with raw device isn't really candid comparison. Ext3 is journaling 
file system which has a big overhead when FW is ON. If one is not 
paranoid  about fs consistency to even consider use of raw device, he 
can use non-journaling file system like ext2 as well which will 
definitely perform much better than ext3. If user wants journaling, he 
can use more "enterprise grade" journaling fs instead ext3 that would 
probably perform much better too.

best regards
Pavel Cisar
IBPhoenix