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Archives For 31 October 2012

Some handy Git commands I often use

10 November 2012 — Leave a comment

Push a specific commit (actually: everything up and including this commit):

git push origin dc97ad23ab79a2538d1370733aec984fc0dd83e1:master

Push everything exept the last commit:

git push origin HEAD~:master

The same, now the last two commits:

git push origin HEAD~2:master

Reorder commits, aka rebasing:

git rebase -i origin

Pulling commits from repo to local

git pull --rebase

When a conflict occurs, solve it, then continue:

git rebase --continue

Put local changes apart (shash them)

git stash save stashname

Show all stashes

git stash list

Retrieve a shash

git stash stashname

When you have committed a change and want to revert it:

Make sure all work is committed or stashed!

git checkout 748796f8f2919de87f4b60b7abd7923adda4f835^ file.pp
git commit
git revert HEAD
git rebase -i
git commit --amend

Explanation:
– Checkout the file as it was before your change (line 1)
– commit it (line 2)
– Revert this commit (line 3)
– Using rebase merge (fixup) this commit with the previous commit that contained a change that you want to remove (line 4)
– Finally, rewrite the commit message and you’re done (line 5)

Git rocks!

In Linux sysadmin
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What a battery means for the performance of our raid array

2 November 2012 — Leave a comment

A few days ago we installed a ‘Battery Backup Unit’ in our secondary storage server. This allows us to turn on the ‘Write Back Cache’. The performance impact was impressive..

Enabling the Write Back Cache means writes are committed to the raid controler’s cache (which is much faster) so you don’t have to wait for the data to be written to disk. Normally, this is a risky operation because when the power goes down unexpectedly the data in the raid controller’s cache is lost. Thanks to the battery the raid controller can finish all of its writes to the disk even there is no more power.

Have a look at the below graph. It shows the load dropped significantly after we’ve installed this battery.

Starting on the left you see normal operations until we switched off the server around midnight. All services kept working by the way, but more about that redundancy magic in another post. The big spike around 1am was caused by syncing the data with the primary storage again after the server came online again. We had not turned on the Write Back Cache at that time. When it finished syncing, we rebooted the server once again, upgraded firmware and activated the Write Back Cache. We immediately saw an performance boost of around 20 times! The small spike around 2am was syncing with the primary storage again, but this time with Write Back Cache enabled. Our load averages now peak at 1 or 2 instead of >20.

Lesson learned: always install a Battery Backup Unit so you can safely turn the Write Back Cache on!

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