[stgt] bad performance maybe blocksize of LUN related

Marcus Schwarz m.schwarz at nwe.de
Fri Aug 12 11:23:45 CEST 2011

Hi there,

> -----Ursprüngliche Nachricht-----
> Von: FUJITA Tomonori [mailto:fujita.tomonori at lab.ntt.co.jp]
> Gesendet: Donnerstag, 11. August 2011 16:29
> An: Marcus Schwarz
> Cc: stgt at vger.kernel.org
> Betreff: Re: bad performance maybe blocksize of LUN related
> You create a file on the file system and use it as an iSCSI logical
> unit. In such case, tgt can't use the native block size
> unconditionally.
> tgt is supposed to specify the block size per lun but seems the
> feature is not working correctly. I'll fix it shortly but meanwhile
> you can the following patch to use the 4096 byte block size
> unconditionally to see if you can get the performance boost.
> diff --git a/usr/sbc.c b/usr/sbc.c
> index cb07be6..0cf40be 100644
> --- a/usr/sbc.c
> +++ b/usr/sbc.c
> @@ -41,7 +41,7 @@
>  #include "spc.h"
>  #include "tgtadm_error.h"
> -#define DEFAULT_BLK_SHIFT 9
> +#define DEFAULT_BLK_SHIFT 12
>  static unsigned int blk_shift = DEFAULT_BLK_SHIFT;

I applied your patch and recompiled the binarys . Now I get 30-40 MB/s in total.

What seems a bit strange to me is that when I start the copy from my local disk to the iscsi device I'm getting around 80 MB/s up until the first GB is transfered. After that is continously dropping the transfer rate until it reaches 30 MB/s .

Changing the MTU to 4k (on iscsi server and windows client) doesn't affect the transfer speed at all.

Do you have a clue what might be the reason for this ? Windows related ?

I'll try to do those transfers from my linux machine again.

Best regards

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