Re: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate

From: Zheng Liu
Date: Wed Apr 18 2012 - 08:41:43 EST

I run a more detailed benchmark again. The environment is as before,
and the machine has Intel(R) Core(TM)2 Duo CPU E8400, 4G memory and a
WDC WD1600AAJS-75M0A0 160G SATA disk.

I use 'fallocate' and 'dd' command to create a 256M file. I compare
three cases, which are fallocate w/o new flag, fallocate w/ new flag,
and dd. We use these commands to create a file. Meanwhile w/ journal
and w/o journal are compared. When I format the filesytem, I use
'-E lazy_itable_init=0' to avoid impact. I use this command to do the

time for((i=0;i<2000;i++)); \
do \
dd if=/dev/zero of=/mnt/sda1/testfile conv=notrunc bs=4k \
count=1 seek=`expr $i \* 16` oflag=sync,direct 2>/dev/null; \

The result:

fallocte dd fallocate w/ new flag
real 0m4.196s 0m3.720s 0m3.782s
user 0m0.167s 0m0.194s 0m0.192s
sys 0m0.404s 0m0.393s 0m0.390s

fallocate dd fallocate w/ new flag
real 1m9.673s 1m10.241s 1m9.773s
user 0m0.183s 0m0.205s 0m0.192s
sys 0m0.397s 0m0.407s 0m0.398s

fallocate dd fallocate w/ new flag
real 1m16.006s 0m18.291s 0m18.449s
user 0m0.193s 0m0.193s 0m0.201s
sys 0m0.384s 0m0.387s 0m0.381s

fallocate dd fallocate w/ new flag
real 1m16.247s 0m18.133s 0m18.417s
user 0m0.187s 0m0.193s 0m0.205s
sys 0m0.401s 0m0.398s 0m0.387s

As result shows, in nojournal mode, the slowdown in w/ conversion case
is not very severe. Obviously it is caused by initializing an unwritten
extent. This patch set aims to reduce this overhead. So we can go on
discussing whether this patch set is acceptable or not. Certainly, if
most of developers strongly object it, just leave it in mailing list.

In journal mode, we can see, when data is set to 'journal', the result
is almost the same. However, when data is set 'ordered' or 'writeback',
the slowdown in w/ conversion case is severe. Then I do the same test
without 'oflag=sync,direct', and the result doesn't change. IMHO, I
guess that journal is the *root cause*. Until now, I don't have a
definitely conclusion, and I will go on traing this issue. Please feel
free to comment it.

Thanks to all who reply the mail. :-)

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