From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bruce Guenter Subject: Re: is space really freed after deleting large subvolume? Date: Fri, 14 Oct 2011 11:12:29 -0600 Message-ID: <20111014171229.GA7528@untroubled.org> References: <20111014084504.GI21931@carfax.org.uk> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="xHFwDpU9dbj6ez1V" To: linux-btrfs@vger.kernel.org Return-path: In-Reply-To: <20111014084504.GI21931@carfax.org.uk> List-ID: --xHFwDpU9dbj6ez1V Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 14, 2011 at 09:45:05AM +0100, Hugo Mills wrote: > Updating and checking the reference count is the thing > that takes time, and can't really be short-circuited. I'm curious if you know how ZFS does this. My boss has used a set of ZFS systems for backup, and indicates that deleting a snapshot is instant, and free space is updated immediately with no apparent background I/O. I know ZFS only has read-only snapshots, but I would think it would have some of the same reference counting challenges. I see reading one of the ZFS presentations that it uses a "birth time" in each block pointer. This lets it optimizes the deletion by walking only a partial set of the nodes. Still, for any non-trivial deletion it would have to walk enough nodes to keep the delete from being as quick as is claimed. --=20 Bruce Guenter http://untroubled.org/ --xHFwDpU9dbj6ez1V Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.17 (GNU/Linux) iQIcBAEBAgAGBQJOmG19AAoJEFFRvcnBTTxnic8P/2S45H9mi7egxBB/NRVLZ2X/ xUEJs0gYueiQfw+SMohl+fSd9285/njL8m/JN1moLS7Mn6Nc1VUVehC7zxFHkL1+ 1Be79H59hCu8aG3IoTjSjUsyjFD6iIhxnbdn73FXIr5tkHVzux6MQUisxA0x6TNV BJnCHNQ/sJVzIs5XghRGk6tJEoZU51pp+VYL1fvzvAc/Zqk62nTYPrH2N2yCeQdF GaCQyTAdUAqmsgw4Omxt2byFPrXv8wRl6zKN4oFwX1gokIwxpTWuEiCrrjkQ2vRU xM7wRlu+OgTwkKjqd1x5LPD2L6azeZOWC/QJTPpcHjzpI5meCof1GVpnZAweMrm6 WKXYb9jvxkqJc6X7s6+E50alDQQdvxVfcEFDEyZp3zs821sOVRv6zqfkIK/mvFrp q5oMgZLlYjqfOFt184z4W5V0sSg8j+0l20i8RgK6wVRkpurneyADPHDd05tPRHf3 U0aYPHUzbWdZzd9Aj8k4c2QXd0vWPNOm3h9VWtQTPmhCUQKZBTXF4n+bkNGIGfCq VKeVAw4a/bAOs1U3I07Q2Z+HrSh9tH++64stZOB+XzXUaLckRHUXfZSdY9S7Y0bu 3G185FGrAXnXwlOQP3s+qrxojOVTFDUQEqHHyFCVXxRj6UVTrpDL7J2ntOHByFhB rAkjuJgXJR9tEAFfRhly =IkUo -----END PGP SIGNATURE----- --xHFwDpU9dbj6ez1V--