Enhancements for nearline tiers in your multi-tier storage pool


Hi again,

With the User Group and IBM TechU this month I have a load of Blog posts that I’ve said I’ll write – so you might see them coming a bit fast this month.

There is a new EasyTier feature that we released in 780 called “Warm Promote”.   This is aiming to address a potential issue when you have a Nearline tier.   I want to stress that I can’t think of a single case where the problem I’m going to describe below caused a major issue for a client – but it’s a good enhancement to the Nearline tier solutions.   And the Warm Promote feature will also help with other scenarios too.

The summary is that if you have a Nearline tier in your storage pool, this feature will probably improve your performance.  That’s all you *need* to know.   However I suspect you’re interested in some details 🙂

What it does

Basically EasyTier is trying to make sure that the data on the Nearline tier is completely idle data.  It might occasionally do some low MB/s workload – but the small block workload should be as close to zero as possible.    Whether it can achieve this obviously depends on whether you have enough space in the higher tier(s) to keep the warm and hot data.

If you have some data that was completely idle yesterday but today you start doing IO to it – then the EasyTier promotion algorithms will spot this and within around 1 day (if there are no other blocking factors) the data will be promoted to a higher tier.   However if this idle data was actually free space in the filesystem, that new data could be a brand new file that you just created and that everyone is using.  Don’t panic, this will not be a disaster, because remember that these Nearline drives are almost entirely idle, so they will almost certainly be able to handle the workload for a single new file.   But we can see that this new file won’t get the best possible performance.

The Warm Promote feature is designed to check every 5 minutes for extents that are no longer idle on the nearline tier, and immediately schedule a promotion to the next tier.   This means that your new file *should* get promoted to a higher tier within 5 or 10 minutes.  This promotion will be scheduled and merged in with all of the other scheduled promotions/demotions/rebalances so it’s not certain to be within 5 minutes, but it will certainly be completed sooner than waiting for the daily promotion schedule.

And all you need to do to turn it on is to simply upgrade to 7.8.0 or higher.   However please note that 7802 is the last 7.8.0 PTF we are planning to release.  So any additional fixes for the 7.8 code stream will only be available in 7.8.1

And one more detail – the Warm Promote feature is only active for the Nearline Tier.  The enterprise Tier is fully capable of running a new workload – especially since the enterprise tier is probably doing very few IOPs to start with.

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