>>>>> Neil Brown (NB) writes: NB> There are a number of aspects to this. NB> - When a write arrives we 'plug' the queue so the stripe goes onto a NB> 'delayed' list which doesn't get processed until an unplug happens, NB> or until the stripe is full and not requiring any reads. NB> - If there is already pre-read active, then we don't start any more NB> prereading until the pre-read is finished. This effectively NB> batches the prereading which delays writes a little, but not too NB> much. NB> - When the stripe-cache becomes full, we wait until it gets down to NB> 3/4 full before allocating another stripe. This means that when NB> some write requests come in, there should be enough room in the NB> cache to delay them until they become full. I see. though my point is a bit different: say, there is an application that's doing big linear writes in order to achieve good throughput. on the other hand, most of modern storages are very sensible to request size and tend to suck serving zillions of small I/Os. raid5 breaks all incoming requests into small ones and handles them separately. of couse, one might be lucky and after submiting those small requests get merged to larger ones. but only due to luck, I'm afraid. what I'm talking about is expressly code in raid5 that would try to merge small requests in some obvious cases. for example: NB> You are right. This isn't optimal. NB> I don't think that the queue should get unplugged at this point. NB> Do you know what is calling raid5_unplug_device in your step 4? NB> We could take the current request into account, but I would rather NB> avoid that if possible. If we can develop a mechanism that does the NB> right thing without reference to the current request, then it will NB> work equally if the request comes down in smaller chunks. note also, that there can be other stripes being served. and they may need reads. thus you'll have to unplug the queue for them. >> cause delayed stripes to get activated. NB> Can you explain where they cause delayed stripes to get activated? just catched it: [<c0106b3e>] dump_stack+0x1e/0x30 [<f881186e>] raid5_unplug_device+0xee/0x110 [ raid5] [<c02452e2>]blk_unplug_work+0x12/0x20 [<c01319ad>] worker_thread+0x19d/0x240 [<c013611a>] kthread+0xba/0xc0 [<c01047c5>] kernel_thread_helper+0x5/0x10 thanks, Alex - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
