Date: Wed, 24 Dec 2014 14:57:44 +0530 From: ashish@FreeBSD.org (Ashish SHUKLA) To: kpneal@pobox.com Cc: Nikos Vassiliadis <nvass@gmx.com>, freebsd-questions@FreeBSD.org Subject: Re: zvol vs plain file for VMs Message-ID: <86zjads8hb.fsf@chateau.d.if> In-Reply-To: <20141223031411.GA752@neutralgood.org> (kpneal@pobox.com's message of "Mon, 22 Dec 2014 22:14:11 -0500") References: <54947594.2060202@gmx.com> <86mw6ilvyf.fsf@chateau.d.if> <549801AF.5000208@gmx.com> <86fvc7my1o.fsf@chateau.d.if> <20141223031411.GA752@neutralgood.org>
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[-- Attachment #1 --] On Mon, 22 Dec 2014 22:14:11 -0500, kpneal@pobox.com said: | On Mon, Dec 22, 2014 at 10:17:31PM +0530, Ashish SHUKLA wrote: || On Mon, 22 Dec 2014 12:34:07 +0100, Nikos Vassiliadis <nvass@gmx.com> said: || || || | On 12/20/14 06:41, Ashish SHUKLA wrote: || || On Fri, 19 Dec 2014 19:59:32 +0100, Nikos Vassiliadis <nvass@gmx.com> said: || || | Hi, || || || || | I have to convert a mail server to a VM and I don't know what kind of || || | storage to use, a plain file or a zvol? || || || || | Are there any pros in using ZVOLs instead of plain files? || || || || | Thanks, Nikos || || || || I don't use FreeBSD for VMs, but theoretically plain file will have filesystem || || overhead which ZVOL won't. || || || || HTH || || || || | Hi, || || | Thank you for your reply Ashish! || || | When you say overhead, you mean overhead on disk or in RAM? could you || | explain a bit more? || || Overhead as in, read/writes to file will go through filesystem driver to block || devices, which involves traversing filesystem structures, compared to ZVOL || which is likely to be contiguous and is exposed as a block device. | Wait, is that correct? I doubt it. | A zvol is created with the 'zfs' command. The man page doesn't say it | allocates space up front, but rather it says it sets a reservation. This | is probably the same meaning of reservation that is used for normal datasets. | It doesn't allocate space on disk, but instead makes a note about basically | quotas for future allocations of space. | Furthermore, for the space on disk to be contiguous it must not be COW. | But ZFS is a COW filesystem. It cannot be both COW and contiguous in use. | Also, if a zvol bypasses the filesystem code then it wouldn't make sense | for zvols to be managed with the 'zfs' command. The zpool command would | make more sense since zpool operates at a lower level than the zfs command. | The practical upshot of all of this is to say that I don't know what the | benefit is, but I also would not put any money on the previous response's | correctness. It can't as simple as described. Sorry, I confused with ZVOLs with LVM LVs, and misunderstood COW's implication. My since apologies for the incorrect reply. Thanks for clarifying it. -- Ashish SHUKLA | GPG: F682 CDCC 39DC 0FEA E116 20B6 C746 CFA9 E74F A4B0 Sent from my Emacs [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBCgAGBQJUmocQAAoJEMdGz6nnT6SwoGUP/17YC74zur2Flv6IA3becBcY /AZQENJSPqKmg+MpUa/t7AQX8/4VsBA/7mBO8Bra45L3O+x3bXaWa+30XbFFt3YP 8qtBl03sSuWjfd+L/Doe42ab73bDNzcYAn6yuBfSfMdIkHw6KMlFCLUrZS38wB89 bbE5ELF+JFNkQbU9g0NE7uZ5C4p6EAHAyt/j7GInNBeFQcsLKTt5NRUW/yHAg983 J00pC07YDtOSuiOriFgjFO131ttlOj4OvxxpXvgDna1Bl0m3oGIJr+7s/ogC/JT7 aGRK1DyZs6QV6GGBOiHLIUKKnJwRrhWZz3TiC84WW8/yJiAMdNap7SjvykOcCDSb 6cNWEZxupDBwcw3j48d0P0QkfWs+Ufm7X1Uw1v+jUMLC+RFHoyMGkEfPBN29zURC QTqZeHMHkqYxbWRB9mEJDSWCbUL15YcXBKf5+U8AHB1ulOjlRAmLfvR185sibhrl SNLjFsULtwWGQerXDaS2yznHJk5bBKIKd3tC+D4CPq75BbnuoFI8OIkU9zIziyJ4 9tv6FXDESNwUCY2dFGTgPyoqQYg+UXJnOwBnBvrBRmcG6GSxpQk6XXaAcHvXh3b4 U+UTP1fLirXkIXiaAnrNZSkXFZ/3dF+M1/rWwicyEpxfA/G896vNimrh6mEqA46B 7xZQjOkAF8SkBuszkNa8 =Jg/u -----END PGP SIGNATURE-----
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