Hi Srinivasa.
I still got page cache increasing even when I used those option.
Here is output without those option:
fio --filename=/opt/test --size=1G --direct=1 --sync=1 --rw=randrw --bs=4k --numjobs=1 --iodepth=8 --runtime=60 --time_based --group_reporting --name=journal-test --invalidate=1 --gtod_reduce=1 --ioengine=libaio
HITS MISSES DIRTIES RATIO BUFFERS_MB CACHE_MB
337 0 0 100.0% 0 95
411 0 89005 100.0% 0 664
337 0 38526 100.0% 0 816
337 0 26605 100.0% 0 920
591 0 19906 100.0% 0 998
337 0 12283 100.0% 0 1046
457 0 11351 100.0% 0 1090
337 0 12398 100.0% 0 1138
337 0 13252 100.0% 0 1190
337 0 11876 100.0% 0 1237
337 0 8723 100.0% 0 1271
337 0 12844 100.0% 0 1321
337 0 4612 100.0% 0 1340
457 0 0 100.0% 0 1340
337 0 3 100.0% 0 1340
457 0 0 100.0% 0 1340
457 0 0 100.0% 0 1340
337 0 0 100.0% 0 1340
336 1 0 99.7% 0 316
And here is output with those option:
fio --filename=/opt/test --size=1G --direct=1 --sync=1 --rw=randrw --bs=4k --numjobs=1 --iodepth=8 --runtime=60 --time_based --group_reporting --name=journal-test --invalidate=1 --gtod_reduce=1 --ioengine=libaio --create_on_open=1 --fallocate=none
HITS MISSES DIRTIES RATIO BUFFERS_MB CACHE_MB
337 0 0 100.0% 0 95
337 0 0 100.0% 0 95
337 0 0 100.0% 0 95
410 1 0 99.8% 0 316
337 0 9 100.0% 0 316
337 0 0 100.0% 0 316
337 0 3 100.0% 0 316
337 0 0 100.0% 0 316
337 0 0 100.0% 0 316
1828 0 10 100.0% 0 316
337 0 0 100.0% 0 316
337 0 0 100.0% 0 316
337 0 0 100.0% 0 316
As you can see the page cache still increase from 95 to 316 when using those option.
I don't understand why the dd command with option oflag=direct gave no page cache increasing while fio increased page cache.
Could you help me please?
Thanks a lot in advance.
Regards,
Son
-----Original Message-----
From: Srinivasa Chamarthy [mailto:chamarthy.raju@xxxxxxxxx]
Sent: Friday, April 21, 2017 12:07 PM
To: Son Chu <son.ct@xxxxxxxxxxxxx>
Cc: Sitsofe Wheeler <sitsofe@xxxxxxxxx>; fio@xxxxxxxxxxxxxxx
Subject: Re: Does fio really bypass page cache with option direct=1?
You may use options --create_on_open=1 --fallocate=none. By default FIO will prepare the file of the size (creates a sparse file) and then performs IO. The options --create_on_open=1 and --fallocate=none should skip the step.
--
Srinivasa R Chamarthy
On Fri, Apr 21, 2017 at 9:54 AM, Son Chu <son.ct@xxxxxxxxxxxxx> wrote:
> Hi Sitsofe,
>
> Sorry if this email troubled you but I could not find any useful
> information for this. I am getting the question with fio page cache
> bypass option (direct=1 ioengine=libaio)
>
> I am using fio version 2.19 with command:
>
> fio --filename=/opt/test --direct=1 --rw=randrw --randrepeat=0
> --ioengine=libaio --bs=4k --rwmixread=75 --iodepth=16 --numjobs=1
> --name=4ktest --size=1G
>
> and i used cachestat tool from here to monitor:
> https://github.com/brendangregg/perf-tools
>
> The output was:
>
> Counting cache functions... Output every 1 seconds.
>
> HITS MISSES DIRTIES RATIO BUFFERS_MB CACHE_MB
>
> 1399 42 0 97.1% 2 31
>
> 1408 0 0 100.0% 2 31
>
> -68699 39 71478 100.1% 3 532
>
> -21325 0 22886 100.0% 3 621
>
> -19956 0 21492 100.0% 3 705
>
> -13739 0 15254 100.0% 3 765
>
> As far as i know the buffer cache and page cache was unified from Linux kernel version 2.4. So i wondered why fio with option direct=1 that didn’t bypass page cache as it should?
>
> I didn’t see page cache increasing when using dd with oflag=direct to copy or write file to disk.
>
> Please correct me if i am wrong.
>
> Thanks a lot in advance.
>
> Regards,
> Son
>
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