ZEVO and USB2 disks? is it fine?

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ZEVO and USB2 disks? is it fine?

Post by si-ghan-bi » Thu Sep 20, 2012 3:58 pm

I found this post online:
http://www.macintouch.com/readerreports ... d14feb2012

It says that ZFS does not play well with USB enclosures. Is it true? This is important to know.
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Re: ZEVO and USB2 disks? is it fine?

Post by shuman » Thu Sep 20, 2012 8:20 pm

I have a 4 bay USB 2 enclosure that I bought several years ago; it was a really cheap one too (off brand). I've used it with Apple's original zfs bits, MacZFS, Zevo (tenscomplement) and now Zevo (greenbytes) and have never had an issue.

-Shuman
- Mac Mini (Late 2012), 10.8.5, 16GB memory, pool - 2 Mirrored 3TB USB 3.0 External Drives
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USB drives with MacZFS and with ZEVO

Post by grahamperrin » Sun Sep 23, 2012 10:48 am

si-ghan-bi wrote:I found this post … 


That 2012 MacInTouch post about MacZFS probably refers to what's detailed in the MacZFS wiki under Issues with USB drives – some precautions there, including a setting that can be applied to disk sleep.

Here with past and present versions of ZEVO, with flash and hard disk drives on USB, I have found no need for those precautions.

----

USB, ZFS and ZEVO aside for a moment: I sometimes find that the disk sleep setting – 

Code: Select all
disk sudo pmset -a disksleep 0


– can effectively prevent issues with a LaCie drive on the FireWire bus (but I prefer to allow sleep and avoid that class of drive).
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Re: ZEVO and USB2 disks? is it fine?

Post by si-ghan-bi » Sun Sep 23, 2012 11:12 am

Maybe a specific model issue? I have since 2008 a LaCie Quadra d2 and I never had any issue with sleep active.
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issue with a LaCie

Post by grahamperrin » Sun Sep 23, 2012 11:59 am

si-ghan-bi wrote:Maybe a specific model issue? …


Possibly. I reported bugs to Apple, and one of the symptoms (FireWire GUID ffffffffffffffff is invalid!) is widely discussed in public, so I treat it as off-topic from ZEVO :-) …

… but for rotational disk drives in general (or with ZEVO), the pmset hint may help to determine whether drive hardware/firmware is contributory to a problem.
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USB 2.0 spun off from various topics

Post by grahamperrin » Tue Apr 16, 2013 11:10 pm

Amongst this forum's discussions of USB 2.0 (or USB 2 or USB2):

  • USB 2.0 hub recommendations
  • in other topics, a significant number of posts suggest not using USB 2.0 for primary disk storage – or words to that effect.

To the best of my knowledge, none of the releases of ZEVO (up to and including Community Edition 1.1.1) were accompanied by caution about USB. So for positioning ZEVO as a solution – is Zevo safe enough? and the like:

  • let's have things as clear as possible under this topic.

From viewtopic.php?p=4606#p4606 :

raattgift wrote:… heavy USB mass storage device class traffic" … if you must, use multiple USB2 busses (i.e. multiple host controllers).


An example. Let's take my current arrangement of a two-disk pool:

Code: Select all
sh-3.2$ zpool status tall
  pool: tall
 state: ONLINE
 scan: scrub repaired 0 in 36h5m with 0 errors on Mon Apr 15 04:31:59 2013
config:

   NAME                                         STATE     READ WRITE CKSUM
   tall                                         ONLINE       0     0     0
     GPTE_78301A52-4AFF-4D96-8DE9-E76ABC14909C  ONLINE       0     0     0  at disk2s2
     GPTE_99056308-F5E2-4314-852C-4DA04732A2D0  ONLINE       0     0     0  at disk4s2

errors: No known data errors
sh-3.2$ date
Wed 17 Apr 2013 04:22:24 BST
sh-3.2$ uptime
 4:22  up  6:10, 4 users, load averages: 8.81 6.67 5.37
sh-3.2$


There's no redundancy of data, which is bad and will be addressed in a few weeks. For this post let's focus on host, bus, dock and hub arrangements with a MacBookPro5,2 (three USB 2.0 ports):

2013-04-17 04-18-44 screenshot.png
2013-04-17 04-18-44 screenshot.png (201.92 KiB) Viewed 119 times


On the same bus/controller as the Sitecom dock, the built-in iSight is rarely used.

I could now move the larger of the pool's two disks (with GPTE_99056308-F5E2-4314-852C-4DA04732A2D0):

  • from the old Sitecom dock, which is limited to two USB 2.0 ports
  • to any point in the 'nest of three-in-one' (Cerulian N14JB, 0x0608 (ten-port USB 2.0 top loading hub)).

Then in the other eight ports of that Cerulian hub, someone might place non-storage devices (printers, cameras et cetera).

… a full buss that has *any* slow device on it, the fast devices experience queues, which may overrun …


I imagine that a flood of traffic through the Cerulian might cause an overrun to affect the larger (GPTE_99056308-F5E2-4314-852C-4DA04732A2D0) part of the pool.

Considerations should include at least:


>> … If ZEVO detects the removal of all the vdev devices …
>> it tries its best to tear down the pool without waiting for I/O …

– I assume that no tear down will be attempted whilst the smaller part of the pool is well-controlled on the other bus.

For myself, I'll not fill all ten ports of the Cerulian hub (the nest of three-in-one) in the way described above. Someone else – say, a newcomer to ZEVO who have never experienced problems with USB 2.0 – might be less cautious.

Two questions

For a pool like that, without redundancy of data, two (of many) possible arrangements:

  1. both disks under one controller where's there's little or no use of the internal iSight (minimal risk of flooding); or
  2. one controller per disk, with significant risk of flooding under just one of the two controllers

– both undesirable! But which one is most likely to cause a problem to the pool?

(Example: a problem that causes a resilver to occur.)

Might failmode=wait reduce the risk of a problem resulting from a flood or choke? If so, post a hint under the zpool failmode … topic.
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Re: ZEVO and USB2 disks? is it fine?

Post by raattgift » Thu Apr 18, 2013 4:03 am

The warning about USB 2 mass storage is generic. It's tolerable for doing highly sequentialized single-threaded accesses, such as storing a movie or watching a movie, or doing a single backup or restore task. For anything multithreaded, it's going to start to hurt because of the protocols involved (in USB 2 the host has to poll the device which makes it hard to gain from the device's buffering and scheduling), and it's going to hurt much worse if seek latency is substantial and the threading shifts access patterns from highly sequential to effectively random.

You can see this most easily by living in a Mac OS X startup volume that is based on a USB 2 disk. Yuck.

ZFS additionally is less tolerant of USB errors, even ones that are normally simply retried by other filesystems.

I would always avoid mounting a USB2-based dataset. "zfs receive -u ..." is fine, as "zfs send". Keep it single threaded. If the dataset is a small number of files and the access pattern will be highly sequential, a USB-2 pool is probably OK. Movies, for instance. For large numbers of files that will be hit somewhat randomly -- a home directory, anything with lots of files subject to com.apple.metadata, and time machine volumes -- I would avoid USB2 entirely. Scrubbing is a highly random I/O pattern, and a scrub of a USB2-based pool is likely to find errors, especially in the presence of any other I/O, but probably those won't be *media* errors, they'll be bus related; this is pretty easy to see if you have an extrernal drive that talks FW and USB2.

Finally, USB 3 is not nearly so bad, although you have to watch out that your device attaches as USB3 and has not fallen back to USB2.
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Re: ZEVO and USB2 disks? is it fine?

Post by shuman » Thu Apr 18, 2013 10:40 am

raattgift wrote:Finally, USB 3 is not nearly so bad, although you have to watch out that your device attaches as USB3 and has not fallen back to USB2.


Can you expand on this? How do I check this?

Thanks.
- Mac Mini (Late 2012), 10.8.5, 16GB memory, pool - 2 Mirrored 3TB USB 3.0 External Drives
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Re: ZEVO and USB2 disks? is it fine?

Post by raattgift » Thu Apr 18, 2013 1:04 pm

USB 3 hubs and devices are pretty much guaranteed to try to fall back to USB 2 in some cases involving bugs, marginal connections, marginal power, unlucky timing at startup, not-wholly-supported/-conformant chipsets, and so forth. It's quite annoying, and cuts across operating systems. (For instance, I have a set of USB 3 external drives that OI variously doesn't see at all or degrades to USB 2; FreeBSD degrades one of them to USB 2 as well; they work fine on Mac OS X10.8.3, except where one hub is involved, in which case the hub has to be power-cycled if the 2012 Mac Mini is power-cycled, or devices will not be probed or will be misprobed as USB 2).

The technology is still fairly new, so this is far from a total surprise. It's also still USB, and USB has various mass-storage problems in practically all software platforms, although 3.0 is, as I said, a vast improvement on its predecessors. :/

The easiest way of finding out how your drives are mounted is from the top left corner of your screen

Applesymbol > About This Mac > More info > System Report > USB

When you open the disclosure triangles, you should see the device under a "USB 3.0 SuperSpeed Bus", otherwise your connection is degraded. USB 3.0 Hi-Speed Bus is not so bad, but USB Hi-Speed Bus [notice the missing 3.0 !] is edging into awful.

PS: nice sql error if you post something with the apple symbol you get from alt-shift-K. :/

PPS: cf. http://en.wikipedia.org/wiki/USB_3.0#Speed_issues
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Re: ZEVO and USB2 disks? is it fine?

Post by shuman » Thu Apr 18, 2013 1:12 pm

Good to know. I have my mirrored pool on two external USB3 enclosures (not on hubs). Now I know of another place to look when performance seems degraded.
- Mac Mini (Late 2012), 10.8.5, 16GB memory, pool - 2 Mirrored 3TB USB 3.0 External Drives
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