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Re: [Xen-users] Cheap IOMMU hardware and ECC support importance

  • To: xen-users@xxxxxxxxxxxxx
  • From: Kuba <kuba.0000@xxxxx>
  • Date: Thu, 03 Jul 2014 10:46:31 +0200
  • Delivery-date: Thu, 03 Jul 2014 08:47:20 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

W dniu 2014-07-03 00:45, lee pisze:

It's tempting to try it out, and I really like the checksumming
it does, and it's also confusing: There's (at least) ZFS and OpenZFS,
and Debian requires you to use fuse if you want ZFS, adding more

You haven't done your research thoroughly enough.

No, I haven't looked into it thoroughly at all.

On Linux there is for all intents and purposes one implementation.

Where is this implementation?  Is it available by default?  I only saw
that there's a Debian package for ZFS which involves fuse.

In case you'd like to try it out, follow these steps:

and just have few minutes of fun. I'm pretty sure a livecd will do. You can also use files instead of real disks.

There's also uncertainty about changes currently being made
to ZFS which makes me wonder if my data might become unreadable after a
software update or a software change when I install the disks in a
different computer --- I've read reports of that happeneing, though it

If you've read about it I'd like to know where.

It was some blog post somewhere --- unfortunately, I can't find it

You don't have to upgrade your pool when you start using newer implementation.

A very long time ago, I lost data with xfs once.  It probably was my own
fault, using some mount parameters wrongly.  That taught me to be very
careful with file system and to prefer file systems that are easy to
use, that don't have many or any parameters that need to be considered
and basically just do what they are supposed to right out of the box.

Does ZFS do that?  Since it's about keeping the data safe, it might have
a good deal of protection against user errors.

Destructive operations are usually called accordingly: zfs destroy, zfs rollback, so they quite clearly express the intention. How can a file system protect you from executing a destructive operation? Snapshots protect you from most user errors. Off-site backups protect you from su errors. To some extent.

It seems that ZFS isn't sufficiently mature yet to use it.  I haven't
learned much about it yet, but that's my impression so far.

As I said above - you haven't done your research very thoroughly.

I haven't, yet all I've been reading so far makes me very careful.  When
you search for "zfs linux mature", you find more sources saying
something like "it is not really mature" and not many, if any, that
would say something like "of course you should use it, it works

"Mature" means different things to different people in different circumstances. Is Linux mature? Is Linux 3.15 mature? If not, is 2.6 mature? Does it mean it has no bugs? If ZoL is not mature enough for you, you can use FreeBSD or Solaris. Or you can use hardware RAID + any other FS. I have the same feeling about ZFS as Gordan - once you start using it, you cannot imagine making do without it. Does it mean you have to use it too? Of course not:) Is it wrong not to use it? Of course not! You should do what _you_ believe is the right thing to do. But try it out nonetheless :) Or try HAMMER (Dragonfly BSD). Or btrfs (although this one probably really is not mature enough).

Well, yes, the disk has failed when it doesn't return data reliably, so
I don't consider that as a problem but as a desirable feature.

What does ZFS do?  Continue to use an unreliable disk?

Until the OS kernel's controller driver decides the disk has stopped
responding and kicked it out.

As far as I've seen, that doesn't happen.  Instead, the system goes
down, trying to access the unresponsive disk indefinitely.

Hence why TLER is still a useful feature
you don't want your application to end up being made to wait for
potentially minutes when the data could be recovered and repaired in a
few seconds if the disk would only give up and return an error in a
timely manner.

So you would be running ZFS on unreliable disks, with the errors being
corrected and going unnoticed, until either, without TLER, the system
goes down or, with TLER, until the errors aren't recoverable anymore and
become noticeable only when it's too late.

ZFS tells you it had problems ("zpool status"). ZFS can also check entire pool for defects ("zpool scrub", you should do that periodically). It's also quite difficult to corrupts the file system itself:

Using ZFS does not mean you don't have to do backups. File system type won't make a difference for a fire inside your enclosure:) But ZFS makes it easy to create backups by replicating your pool or datasets ("zfs send" lets you create full or incremental backups) to another set of disks or machine(s).

You seem to like the HGST ones a lot.  They seem to cost more than the
WD reds.

I prefer them for a very good reason:

Those guys don't use ZFS.  They must have very good reasons not to.

They do:

And I believe they have lots of good reasons to do so :)


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