From: Eric Sandeen Subject: Re: [PATCH] e2fsprogs: don't set stripe/stride to 1 block in mkfs Date: Tue, 05 Apr 2011 09:39:07 -0700 Message-ID: <4D9B45AB.8000208@redhat.com> References: <4D9A17F8.4000406@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: ext4 development , Zeev Tarantov To: Andreas Dilger Return-path: Received: from mx1.redhat.com ([209.132.183.28]:59099 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755287Ab1DEQjN (ORCPT ); Tue, 5 Apr 2011 12:39:13 -0400 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On 4/5/11 1:10 AM, Andreas Dilger wrote: > On 2011-04-04, at 9:11 AM, Eric Sandeen wrote: >> Block devices may set minimum or optimal IO hints equal to >> blocksize; in this case there is really nothing for ext4 >> to do with this information (i.e. search for a block-aligned >> allocation?) so don't set fs geometry with single-block >> values. >> >> Zeev also reported that with a block-sized stripe, the >> ext4 allocator spends time spinning in ext4_mb_scan_aligned(), >> oddly enough. >> >> Reported-by: Zeev Tarantov >> Signed-off-by: Eric Sandeen >> --- >> >> diff --git a/misc/mke2fs.c b/misc/mke2fs.c >> index 9798b88..74b838c 100644 >> --- a/misc/mke2fs.c >> +++ b/misc/mke2fs.c >> @@ -1135,8 +1135,11 @@ static int get_device_geometry(const char *file, >> if ((opt_io == 0) && (psector_size > blocksize)) >> opt_io = psector_size; >> >> - fs_param->s_raid_stride = min_io / blocksize; >> - fs_param->s_raid_stripe_width = opt_io / blocksize; >> + /* setting stripe/stride to blocksize is pointless */ >> + if (min_io > blocksize) >> + fs_param->s_raid_stride = min_io / blocksize; >> + if (opt_io > blocksize) >> + fs_param->s_raid_stripe_width = opt_io / blocksize; > > I don't think it is harmful to specify an mballoc alignment that is > an even multiple of the underlying device IO size (e.g. at least > 256kB or 512kB). > > If the underlying device (e.g. zram) is reporting 16kB or 64kB opt_io > size because that is PAGE_SIZE, but blocksize is 4kB, then we will > have the same performance problem again.> > Cheers, Andreas I need to look into why ext4_mb_scan_aligned is so inefficient for a block-sized stripe. In practice I don't think we've seen this problem with stripe size at 4 or 8 or 16 blocks; it may just be less apparent. I think the function steps through by stripe-sized units, and if that is 1 block, it's a lot of stepping. while (i < EXT4_BLOCKS_PER_GROUP(sb)) { ... if (!mb_test_bit(i, bitmap)) { ... } i += sbi->s_stripe; } But in any case, setting stripe alignment to 1 block makes no sense to me, and I see no reason to do it at mkfs time... -Eric