From: Eric Sandeen <sandeen@redhat.com>
To: "Wang, Zhiye" <Zhiye.Wang@Arcserve.com>,
"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: Questions about FIEMAP
Date: Mon, 12 Oct 2015 08:27:13 -0500 [thread overview]
Message-ID: <561BB531.6070706@redhat.com> (raw)
In-Reply-To: <CY1PR10MB04283CC6BA7D9AE8F3F0B984E4310@CY1PR10MB0428.namprd10.prod.outlook.com>
On 10/11/15 11:37 PM, Wang, Zhiye wrote:
> Hello everyone,
>
> After googled a bit, I got information that btrfs supports FIEMAP (as "cp" needs it), but it's not valid for "write" operation.
cp should not be using fiemap any more. It was for a while, until they realized that copying based on fiemap output could lead to corruption because things changed between the fiemap call and the actual copy...
> I guess we cannot write to block device directly after get block list using FIEMAP. This is because:
>
> 1. COW feature of btrfs (but this can be disabled using NOCOW)
> 2. File system rebalance
> 3. Defragmentation
>
> Aren't item #2 and #3 also a problem for "read" operation? For example, after "cp" get block list using FIEMAP, file system rebalance occurs, So, previous result of FIEMAP is not valid anymore.
>
> Or maybe I misunderstood something. Please correct me.
That all may be true for btrfs, but more fundamentally as dsterba said, nothing guarantees that the layout won't change *immediately* after your fiemap call. This is the case on any filesystem, not just btrfs.
-Eric
next prev parent reply other threads:[~2015-10-12 13:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-12 4:37 Questions about FIEMAP Wang, Zhiye
2015-10-12 6:01 ` Duncan
2015-10-12 8:10 ` Hugo Mills
2015-10-12 13:04 ` David Sterba
2015-10-12 13:27 ` Eric Sandeen [this message]
2015-10-28 9:57 ` Wang, Zhiye
2015-10-28 12:36 ` Duncan
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