Daily Archives: July 17, 2012

RegEx to match a substring after a delimiter

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They say that if you have a problem and want to use RegEx to solve it, then you have two problems. So true! 🙂

My specific problem was that I wanted to search for a string within a substring after a delimiter sign, more precisely, in the last segment of a path. Here is an example:

/some/test_path/to/search/with_a_Test_file.txt

The RegEx, searching without case sensitivity for “test” should return a match only for the portion of the string after the last “/”.
All suggestions, which I could find on StackOverflow, concerned with matching the entire file name and not a portion of it, so I had to learn some advanced RegEx. Fast.

The answer was something, called “lookahead”, which is well explained at Regular-Expressions.info site.

The resulting RegExt string looks like some serious swearing in a cartoon bubble… 🙂 Here is the code, which is accepted by PHP’s preg_match() function:

/(?=[^\/]+$)test/i

According to my (rather limited) understanding of RegEx, the first portion in the parenthesis, after the “?=”is the lookahead, which matched the entire file name after the last “/”, then comes the search substring, “test”, which operates on that result and, finally, “/i” is the switch, instructing a case-insensitive match.

Adding disks by label in ZFS and making those labels stick around

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When I stared building my new file server, I decided to add the disks to ZFS vdevs by label and not by the device id, i.e:

#glabel label l1 /dev/ada0
#glabel label l2 /dev/ada1

After a reboot, those labelled disks suddenly started to show up as /dev/ada0 and /dev/ada1 again and the labels disappeared from /dev/label directory.

For the existing disks, I tried to offline each disk in turn and re-label it. A new problem turned up then: I could not replace the /dev/adaX offlined disks with the same labelled ones, as zpool gave an error of the device “is part of active pool”.

After some further searching, I found out that I had to zero out the first and the last megabyte of the disk before labelling it and replacing in zpool:

#dd if=/dev/zero of=/dev/ada0 bs=1m count=1
#dmesg | grep ada0
<read the block count value, subtract 2048 and provide the result to the seek switch below>
#dd if=/dev/zero of=/dev/ada0 seek=358746954
#glabel label l1 /dev/ada0
#zpool replace zstore /dev/ada0 label/l1

At this point zpool status was again showing labels. However, after the next reboot, the labels were gone again and I was pretty frustrated. Back to the search engine.

On page 3 of some discussion of this matter, I noticed two additional steps, which should fix the problem. After performing the steps above and re-labelling and re-placing the disks, I issued:

#zpool export zstore
#zpool import -d /dev/label zstore

The -d switch is what instructs zpool to read the disk references from a specific directory and it makes the labels stick around.

When I added subsequent new disks to the pool, I followed these steps to make the labels stick and to avoid re-labelling at a later point:

  1. Zero-out the first and the last part of each disk that will comprise the new vdev (especially important if the disk has been in use before and does not come staight from the factory)
  2. Label each disk with glabel
  3. #zpool add zstore raidz label/l5 label/l6 etc….
  4. #zpool export zstore
  5. #zpool import -d /dev/label zstore

And the labels never disappeared again.

This same procedure can be applied to labelling your ZIL and LARC devices.