Date: Wed, 3 Jul 2013 01:34:36 -0500 From: Kevin Day <toasty@dragondata.com> To: Jeremy Chadwick <jdc@koitsu.org> Cc: freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: EBS snapshot backups from a FreeBSD zfs file system: zpool freeze? Message-ID: <6488DECC-2455-4E92-B432-C39490D18484@dragondata.com> In-Reply-To: <20130703055047.GA54853@icarus.home.lan> References: <87li5o5tz2.wl%berend@pobox.com> <CA%2BtpaK1jQuKneQsxkVfxJGzXdPdLZfqBM1QWQ0e19nK5t71t1Q@mail.gmail.com> <87ehbg5raq.wl%berend@pobox.com> <20130703055047.GA54853@icarus.home.lan>
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--Apple-Mail=_E9F25EA9-6758-4A9C-A380-FD064D1B1DB1 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii On Jul 3, 2013, at 12:50 AM, Jeremy Chadwick <jdc@koitsu.org> wrote: > On Wed, Jul 03, 2013 at 02:08:13PM +1200, Berend de Boer wrote: >>=20 >> For starters, I suppose very few people are using FreeBSD on AWS, so >> "most of us" don't have a choice :-) >>=20 >> But this might simply be my understanding: what if I want to use the >> EBS snapshot of the 5 disks I've taken and attach them to another >> machine, and mount it? >>=20 >> But perhaps you don't know what an EBS snapshot is? It's not a backup >> of your file system, it's a hardware-based backup of a disk at a >> single point in time. >>=20 >> I.e. with EBS I can take a snapshot of 5 1TB, create new disks from >> it, and attach it to another machine. In SECONDS. >=20 > Okay, I think I understand what you're asking. Please correct me: >=20 > It sounds to me like the Linux OS images on AWS have utilities or the > capability to create EBS images that are snapshots of the "virtual > disks" that make up the AWS system, and that you can transfer these > to another Linux AWS machine and mount the EBS images, and that this = is > being done within Linux itself. >=20 > Correct? >=20 > If so -- then what you're wanting to ask is: "does FreeBSD have = support > for EBS images?" (Meaning this has nothing to do with ZFS) I get the > impression an EBS image is a proprietary Amazon thing, so you would = need > to ask Amazon if they have the same utilities for FreeBSD, or ask the > individual(s) responsible for the FreeBSD AWS images if there are such > tools. Again: nothing to do with ZFS. Not quite, we run into something similar with VMware ESXi, so let me try = to rephrase it. ESXi provides block storage to virtual machines that appear to be = traditional disks. ESXi has no real knowledge of the contents, files, = it's just dumb block storage. This is like EBS, the volumes appear to be = normal disks to the virtual machine. But, the backend storage system allows you to take snapshots of the = virtual disks. You can then roll back to an earlier snapshot, or make = copies of the snapshots for other VMs to use. The problem is that unless = you quiesce the filesystem before making the snapshot, the contents are = potentially "dirty". It could be mid-write, or in the middle of doing = something that would be bad to leave that way. If you're unlucky and = make a snapshot during one of those inconsistent periods, if you were to = rollback or clone that snapshot you'd get fsck complaining (for UFS) or = ZFS may need to do some repairing, etc. This is much less an issue with = ZFS than other filesystems, but it's still possible to think you've = written something, try to trigger a snapshot, and it's not actually = committed to disk yet. Linux has an ioctl(?) you can call that says "Quiesce this filesystem", = where all dirty buffers are flushed and (i believe), writes are blocked, = and the filesystem is temporarily marked clean. You then trigger the = backend storage to make a filesystem, then you unfreeze the filesystem = so that normal I/O can continue. The closest thing we can do in FreeBSD is to unmount the filesystem, = take the snapshot, and remount. This has the side effect of closing all = open files, so it's not really an alternative.=20 The other option is to not freeze the filesystem before taking the = snapshot, but again you risk leaving things in an inconsistent state, = and/or the last few writes you think you made didn't actually get = committed to disk yet. For automated systems that create then clone = filesystems for new VMs, this can be a big problem. At best, you're = going to get a warning that the filesystem wasn't cleanly unmounted. --Apple-Mail=_E9F25EA9-6758-4A9C-A380-FD064D1B1DB1 Content-Disposition: attachment; filename=smime.p7s Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIIPLzCCBN0w ggPFoAMCAQICEHGS++YZX6xNEoV0cTSiGKcwDQYJKoZIhvcNAQEFBQAwezELMAkGA1UEBhMCR0Ix GzAZBgNVBAgMEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBwwHU2FsZm9yZDEaMBgGA1UECgwR Q29tb2RvIENBIExpbWl0ZWQxITAfBgNVBAMMGEFBQSBDZXJ0aWZpY2F0ZSBTZXJ2aWNlczAeFw0w NDAxMDEwMDAwMDBaFw0yODEyMzEyMzU5NTlaMIGuMQswCQYDVQQGEwJVUzELMAkGA1UECBMCVVQx FzAVBgNVBAcTDlNhbHQgTGFrZSBDaXR5MR4wHAYDVQQKExVUaGUgVVNFUlRSVVNUIE5ldHdvcmsx ITAfBgNVBAsTGGh0dHA6Ly93d3cudXNlcnRydXN0LmNvbTE2MDQGA1UEAxMtVVROLVVTRVJGaXJz dC1DbGllbnQgQXV0aGVudGljYXRpb24gYW5kIEVtYWlsMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A MIIBCgKCAQEAsjmFpPJ9q0E7YkY3rs3BYHW8OWX5ShpHornMSMxqmNVNNRm5pELlzkniii8efNIx 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