Re: REPORT: FIO random read performance degradation without "norandommap" option

["Kudryavtsev, Andrey O" <andrey.o.kudryavtsev@intel.com>]

Robert, thanks for the reply.
That�s exactly confirms my findings. Any idea if LBA map is generated for
every job in case of multiple numjobs?

The fact of the performance recovery after degradation was completely
confusing too. I�m glad the solution was so easy.

Jens, are there any chance to add the note into the documentation?

-- 
Andrey Kudryavtsev,

SSD Solution Architect
Intel Corp. 



On 1/9/15, 2:49 PM, "Elliott, Robert (Server Storage)" <Elliott@hp.com>
wrote:

>
>> -----Original Message-----
>> From: fio-owner@vger.kernel.org [mailto:fio-owner@vger.kernel.org] On
>> Behalf Of Kudryavtsev, Andrey O
>> Sent: Friday, 09 January, 2015 4:42 PM
>> To: fio@vger.kernel.org
>> Subject: REPORT: FIO random read performance degradation without
>> "norandommap" option
>>
>> Colleagues,
>> I executed 2-hour runs of 4KRR to understand performance changes across
>> the time on the specific very fast NVMe SSD with 1.6TB capacity.
>> I noticed the side effect of �norandommap� parameter performing full
>>span
>> test on the block device.
>> Here is the example of the result with random map (I.e. without
>> "norandommap" option) within 120 minutes windows.
>> [cid:E6872B64-35D1-4447-A0CF-32E6411D9BDB]
>> (IOPS in blue)
>>
>> As soon as I enabled �norandommap� option the curve has changed into the
>> straight line as expected.
>
>It takes resources to maintain the random map table.  I always run with
>norandommap unless using verify, which has to remember which ones have
>been accessed.
>
>Here's a description I gave someone a while back:
>With a huge device (e.g., 5.8 TB from RAID-0 made from 16 SSDs),
>if you do not use "norandommap", fio allocates a bitmap for all
>the disk blocks to keep track of where it has read or written.
>It uses this to avoid accessing the same blocks until all
>the blocks have been accessed, and to know which blocks it
>needs to verify if verify=<something> is enabled.
>
>For 5.8 TB, that is 1562714136 = 1.5 GB.  Not many of those
>huge allocations work, so it
>* hangs the system for a while
>* generates estimates like
>        [eta 1158050440d:06h:50m:22s]
>* and eventually reports
>        smalloc: failed adding pool
>        fio: failed allocating random map. If running a large number of
>jobs, try the 'norandommap' option or set 'softrandommap'. Or give a
>larger --alloc-size to fio.
>
>fio continues to run after that; I think it verifies only
>the devices for which the allocation worked and ignores
>the rest.
>
>
>---
>Rob Elliott    HP Server Storage
>
>