Subject: Re[2]: [PATCH 03/11][v3] async_tx: add support for asynchronous RAID6 recovery operations

From: Yuri Tikhonov <[email-masked]>
Date: Fri, 16 Jan 2009 14:51:42 +0300

On Thursday, January 15, 2009 Dan Williams wrote:

> On Mon, Jan 12, 2009 at 5:43 PM, Yuri Tikhonov <[email-masked]> wrote:
>> +       /* (2) Calculate Q+Qxy */
>> +       lptrs[0] = ptrs[failb];
>> +       lptrs[1] = ptrs[disks-1];
>> +       lptrs[2] = NULL;
>> +       tx = async_pq(lptrs, NULL, 0, 1, bytes, ASYNC_TX_DEP_ACK,
>> +                     tx, NULL, NULL);
>> +
>> +       /* (3) Calculate P+Pxy */
>> +       lptrs[0] = ptrs[faila];
>> +       lptrs[1] = ptrs[disks-2];
>> +       lptrs[2] = NULL;
>> +       tx = async_pq(lptrs, NULL, 0, 1, bytes, ASYNC_TX_DEP_ACK,
>> +                     tx, NULL, NULL);
>> +

> These two calls convinced me that ASYNC_TX_PQ_ZERO_{P,Q} need to go.
> A 1-source async_pq operation does not make sense.

 Another source is hidden under not-set ASYNC_TX_PQ_ZERO_{P,Q} :) 
Though, I agree, this looks rather misleading.

>   These should be:

>        /* (2) Calculate Q+Qxy */
>        lptrs[0] = ptrs[disks-1];
>        lptrs[1] = ptrs[failb];
>        tx = async_xor(lptrs[0], lptrs, 0, 2, bytes,
>                       ASYNC_TX_XOR_DROP_DST|ASYNC_TX_DEP_ACK, tx, NULL, NULL);

>         /* (3) Calculate P+Pxy */
>        lptrs[0] = ptrs[disks-2];
>        lptrs[1] = ptrs[faila];
>        tx = async_xor(lptrs[0], lptrs, 0, 2, bytes,
>                       ASYNC_TX_XOR_DROP_DST|ASYNC_TX_DEP_ACK, tx, NULL, NULL);

 The reason why I preferred to use async_pq() instead of async_xor() 
here is to maximize the chance that the whole D+D recovery operation 
will be handled in one ADMA device, i.e. without channels switch and 
the latency introduced because of that.

 So, if we'll decide to stay with ASYNC_TX_PQ_ZERO_{P,Q}, then this 
should be probably kept unchanged, but if we'll get rid of 
ASYNC_TX_PQ_ZERO_{P,Q}, then, obviously, we'll have to use 
async_xor()s here as you suggest.

 Regards, Yuri

 --
 Yuri Tikhonov, Senior Software Engineer
 Emcraft Systems, www.emcraft.com