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Old 09-30-2011, 04:03 PM
Lamar Owen
 
Default add on sata card relabeling drives, installation

On Friday, September 30, 2011 11:41:02 AM Les Mikesell wrote:
> Because I can. Why wouldn't you?
...
> That doesn't any more sense than having to label all your shipping
> containers descriptively before you know what you are going to put in
> them. And besides, most of the labels are applied by the installer
> without user input.

While finding the corner cases seems to be your specialty, Les, recognize that there will always be a corner case not covered by any filesystem labeling/naming scheme, no matter what scheme is used.

It's not at all hard to change the labels after the install.

Linux disk device names aren't reliable (with iSCSI and other SAN technologies they never have been).

LVM has name collision issues (if two sets of one volume group name are found, hilarity ensues, part of the reason why LVM volume group names picked by the installer are now in EL6 based on hostname and not just generic names as before).

Labels of course have their own collision issues, but a label is the one thing that is the most easily modified by the user; use the chosen filesystem's labeling command (e2label for ext2/3/4; other filesystems have their own) and change it in /etc/fstab as well; next reboot it will get picked up. Labels have serious multipathing issues.

UUID is, IMHO at least, the worst of all worlds due to the length and the user-unfriendliness of it all (it's been the Ubuntu default for a while, though!). It is guaranteed unique (until you use complete clones), but is the most difficult to change and use.

Doing it by controller, channel, and logical unit makes a lot of sense until you change things around a few times (and with SAN technologies change is very easy). My boot drive on one box gets a new drive 'letter' (yuck, DOSism at its worst!) nearly every boot due to the highly dynamic and multipathed nature of of the SAN fabric connection to it, and the fibre channel HBA is being enumerated before the boot RAID controller (3Ware).

And it needs to be that way because of the different PCI-X speeds involved, as well as cable lengths and clearance issues inside the 2U server's chassis. But having /dev/sdag as my boot drive doesn't bother me in the least; everything is either LVM or label-based mounting, and I haven't had any collision problems (but multipath problems are a different story).

But my multipathing issues relate to my situation being one of those corner cases (the normal multipathing assumes an A and a B side redundancy from HBA ports through the fabric to the storage processors to the backend loops; while I will soone be there I am not right now, with machines seeing four paths to every LUN and not just two). When I get things into the recommended dual-path HA state either the standard EL-provided multipathing or the EMC PowerPath routing will work as designed, so I can't actually complain that my situation is working properly.
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Old 09-30-2011, 04:20 PM
Benjamin Franz
 
Default add on sata card relabeling drives, installation

On 9/30/2011 8:41 AM, Les Mikesell wrote:
> On Fri, Sep 30, 2011 at 4:56 AM, Hakan Koseoglu<hakan@koseoglu.org> wrote:
>> Why would you move disks around machines unless you're recovering them
>> after a failure?
> Because I can. Why wouldn't you? Mine are nearly all in swappable
> carriers and it is a lot faster to move them than to ship data any
> other way.
>

Because you are wearing the machine's connectors out. They are rated to
be *infrequently* changed out. When you do it on a regular basis it will
just be a matter of time until they develop electrical/physical problems.

If you want to use drives to ship data around plug in a USB hub and
connect USB drives to it. That way when the connectors inevitably wear
out all you need to replace is the hub (and/or the drives).

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Old 09-30-2011, 04:26 PM
Les Mikesell
 
Default add on sata card relabeling drives, installation

On Fri, Sep 30, 2011 at 11:03 AM, Lamar Owen <lowen@pari.edu> wrote:
>
> While finding the corner cases seems to be your specialty, Les, recognize that there will always be a corner case not covered by any filesystem labeling/naming scheme, no matter what scheme is used.
>

I've found that it is a good idea to find concepts and implementations
that weren't very well thought out before they bite you somewhere, so
yes, I do go out of my way. For example when mounting by label was
first implemented, having a duplicate label (very likely if you move
disks around at all since the installer always used the same labels)
would keep the system from booting at all. You had to just say 'what
were they thinking...' - and wonder about the rest of the system.

> It's not at all hard to change the labels after the install.

To what? It's something that is going to hold some data in the
future. And you may not know you need to re-mount it until the
machine that labeled it is gone or dead and the drive is all that is
left.

Within 5.x I've found auto-assembled md devices to be pretty reliable
at identifying themselves, but booting the 6.x livecd completely
messed that up on the one machine where I tried it.

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Old 09-30-2011, 04:43 PM
Les Mikesell
 
Default add on sata card relabeling drives, installation

On Fri, Sep 30, 2011 at 11:20 AM, Benjamin Franz <jfranz@freerun.com> wrote:
>
>>> Why would you move disks around machines unless you're recovering them
>>> after a failure?
>>
>> Because I can. *Why wouldn't you? * Mine are nearly all in swappable
>> carriers and it is a lot faster to move them than to ship data any
>> other way.
>>
>
> Because you are wearing the machine's connectors out. They are rated to be
> *infrequently* changed out. When you do it on a regular basis it will just
> be a matter of time until they develop electrical/physical problems.

Source? The numbers I've seen are on the order of 50,000 insertions.

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Old 09-30-2011, 04:57 PM
Lamar Owen
 
Default add on sata card relabeling drives, installation

On Friday, September 30, 2011 12:26:28 PM Les Mikesell wrote:
> On Fri, Sep 30, 2011 at 11:03 AM, Lamar Owen <lowen@pari.edu> wrote:
> >

> For example when mounting by label was
> first implemented, having a duplicate label (very likely if you move
> disks around at all since the installer always used the same labels)
> would keep the system from booting at all. You had to just say 'what
> were they thinking...' - and wonder about the rest of the system.

Again I'll say that no matter what scheme were to be used there are issues and problems. 'What were they thinking?' is something that obviously the nameless 'they' must answer for themselves, but at the same time I'm reminded of the old engineering adage 'the better is the enemy of the good enough' meaning that while you can always make a product 'better' you must recognize when it is good enough for the targeted use case. And if your particular corner case is not the targeted use case... well, things do break. Try not to have known corner cases or be prepared to work around the breakage.

But 'breakage' and 'bugginess' are not synonyms; something can be broken for a corner case but not be a bug in the general sense. Is the current filesystem mounting standard broken? In certain use cases most certainly. Is the current filesystem mounting standard buggy? For the targeted use cases probably not. After all, upstream developers and CentOS builders all operate within finite resource limits; it takes infinite resources to reach perfection.

> > It's not at all hard to change the labels after the install.
>
> To what? It's something that is going to hold some data in the
> future. And you may not know you need to re-mount it until the
> machine that labeled it is gone or dead and the drive is all that is
> left.

The only truly unique identifier belonging to the drive and externally visible is the drive serial number. Or you can literally and physically label the disk with information about its filesystems; I've both seen that done and have done it in certain hotswap cases.

> Within 5.x I've found auto-assembled md devices to be pretty reliable
> at identifying themselves, but booting the 6.x livecd completely
> messed that up on the one machine where I tried it.

There seem to be enough differences in the md scheme of 5.x and 6.x to discourage disk interchange among the two in mdraid cases. Having said that, I have an EL6.1 (upstream EL) machine with this:
[root@www ~]# cat /proc/mdstat
Personalities : [raid1]
md127 : active raid1 sdae1[0] sdaf1[1]
732570841 blocks super 1.2 [2/2] [UU]

unused devices: <none>
[root@www ~]#

Yeah, md127. But it works reliably, so why change it? When LUNs go away or whatnot, the member disks, between boots, will change in terms of device name (for a while they were /dev/sdw and /dev/sdx, then I added some LUNs to the fibre channel and they went to /dev/sdz and /dev/sdaa; I've added a LUN or two since then (and thanks to multipathing) they are now at /dev/sdae and /dev/sdaf; the mirror hasn't broken. And this md set was created under CentOS 5 a couple of years back.

This would definitely break things if mount points in /etc/fstab are keyed by md number; that's not the case here, the filesystem is mounted by label.

But is that buggy? Depends entirely on use case.
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Old 09-30-2011, 05:55 PM
 
Default add on sata card relabeling drives, installation

Benjamin Franz wrote:
> On 9/30/2011 8:41 AM, Les Mikesell wrote:
>> On Fri, Sep 30, 2011 at 4:56 AM, Hakan Koseoglu<hakan@koseoglu.org>
>> wrote:
>>> Why would you move disks around machines unless you're recovering them
>>> after a failure?
>> Because I can. Why wouldn't you? Mine are nearly all in swappable
>> carriers and it is a lot faster to move them than to ship data any
>> other way.
>>
> Because you are wearing the machine's connectors out. They are rated to
> be *infrequently* changed out. When you do it on a regular basis it will
> just be a matter of time until they develop electrical/physical problems.
<snip>
Most of our servers have all drives in hot swap bays (and the older ones
that don't are being surplussed as fast as we can)... *ALL* of which have
sleds they have to fit in. The only drives I swap on a regular basis are
our offline backups (of the online backups), and that's every two weeks,
and for that I've got a dual bay eSATA base, just drop them in, then push
it up. Nothing else moves until it dies.

mark

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Old 09-30-2011, 06:12 PM
Les Mikesell
 
Default add on sata card relabeling drives, installation

On Fri, Sep 30, 2011 at 12:55 PM, <m.roth@5-cent.us> wrote:
>
> Most of our servers have all drives in hot swap bays (and the older ones
> that don't are being surplussed as fast as we can)... *ALL* of which have
> sleds they have to fit in. The only drives I swap on a regular basis are
> our offline backups (of the online backups), and that's every two weeks,
> and for that I've got a dual bay eSATA base, just drop them in, then push
> it up.

If 750gb disks are big enough, you can get a cute little internal
trayless hot swap bay for 2 - 2.5" SATA drives that fits in the space
a 3.5" floppy would have taken. The WD 'Scorpio Black" drives are
pretty snappy - and you can toss your backup in your shirt pocket. I
think someone even has a 1 Tb drive in the standard laptop height now.
Until recently there were 2.5" 1 and 1.5 Tb drives but they were too
tall for standard enclosures.

--
Les Mikesell
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Old 09-30-2011, 06:39 PM
 
Default add on sata card relabeling drives, installation

Les Mikesell wrote:
> On Fri, Sep 30, 2011 at 12:55 PM, <m.roth@5-cent.us> wrote:
>>
>> Most of our servers have all drives in hot swap bays (and the older ones
>> that don't are being surplussed as fast as we can)... *ALL* of which
>> have sleds they have to fit in. The only drives I swap on a regular
>> basis are our offline backups (of the online backups), and that's every
>> two weeks, and for that I've got a dual bay eSATA base, just drop them
>> in, then push it up.
>
> If 750gb disks are big enough, you can get a cute little internal
> trayless hot swap bay for 2 - 2.5" SATA drives that fits in the space
> a 3.5" floppy would have taken. The WD 'Scorpio Black" drives are
> pretty snappy - and you can toss your backup in your shirt pocket. I
> think someone even has a 1 Tb drive in the standard laptop height now.
> Until recently there were 2.5" 1 and 1.5 Tb drives but they were too
> tall for standard enclosures.

#insert "rocking_chair.h"
Why, Ah remember when my bosses at a job long ago gave me a *big*, brand
new drive as a holiday gift, knowing I'd be working at home. Why, it was
all of 30MB!

Sorry, the 750GB's are going, going, gone, and even the 1TB's are
"smaller", except for the HPC clusters, which don't need a lot of disk.
And no, I do not toss my backups at work in my shirt: the offline backups,
fully encrypted disks, go in the fire safe in the locked server room. Some
of the systems they're backing up have HIPAA and PII data.

mark "why, yes, I *am* where some of your US tax dollars
are going"

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Old 09-30-2011, 07:04 PM
Les Mikesell
 
Default add on sata card relabeling drives, installation

On Fri, Sep 30, 2011 at 1:39 PM, <m.roth@5-cent.us> wrote:
>
> And no, I do not toss my backups at work in my shirt:

Figuratively speaking, of course - the 2.5" drives are just easier for
any use where the capacity makes sense.

> the offline backups,
> fully encrypted disks, go in the fire safe in the locked server room. Some
> of the systems they're backing up have HIPAA and PII data.

Do you encrypt at the disk level (hardware support?), the filesystem,
or just per file with tar-type files? And has the technique ever
caused data loss?

--
Les Mikesell
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Old 09-30-2011, 07:27 PM
 
Default add on sata card relabeling drives, installation

Les Mikesell wrote:
> On Fri, Sep 30, 2011 at 1:39 PM, <m.roth@5-cent.us> wrote:
>>
>> And no, I do not toss my backups at work in my shirt:
>
> Figuratively speaking, of course - the 2.5" drives are just easier for
> any use where the capacity makes sense.
>
Um, we just went from the 1TB drives to 3TB drives, so I only need four.

>> the offline backups,
>> fully encrypted disks, go in the fire safe in the locked server room.
>> Some of the systems they're backing up have HIPAA and PII data.
>
> Do you encrypt at the disk level (hardware support?), the filesystem,
> or just per file with tar-type files? And has the technique ever
> caused data loss?

As I said, the disks are fully encrypted, using LUKS. Can't do hardware
support - these are just ordinary drives. Never had data loss, unless the
drive started failing.

mark

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