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XFS: the filesystem of the future?

XFS: the filesystem of the future?

Posted Jan 30, 2012 17:35 UTC (Mon) by Otus (subscriber, #67685)
In reply to: XFS: the filesystem of the future? by sbergman27
Parent article: XFS: the filesystem of the future?

> Drive caches are a small factor in comparison. We didn't really even used to worry, or even thing about them.

They used to be small. They've grown approximately at the same speed as HDD sizes, which is significantly faster than throughput, not to mention seek time.

A consumer HDD from c. 2000 might have a 2 MB cache and 40 MB/s throughput, so a full cache empties from sequential data in 50 ms best case. Current 2-3 TB drives have a 64 MB cache and 100-150 MB/s throughput, so a full cache takes around 500 ms minimum to empty.

For non-sequential data it's much worse.


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XFS: the filesystem of the future?

Posted Feb 2, 2012 16:57 UTC (Thu) by jd (guest, #26381) [Link]

Cache would not be a problem if:

(a) it was battery-backed, and
(b) was write-through

Battery-backed doesn't have to mean the whole drive has to remain powered-up, it just has to mean the DRAM gets enough juice to keep refreshing until regular power is restored *if* there is any unwritten content in it. In other words, if everything is flushed to disk then you don't need to keep the drive's RAM powered. If drive manufacturers were *really* clever, then only those blocks of RAM with unflushed content would need to remain powered.

It's hard to get a frame of reference, as most devices with RAM and a modern LIon battery also have a power-hungry CPU and an even hungrier RF system to feed. Here, you only need to keep selective RAM chips powered, no processing is required. I have absolutely no idea what kind of leakage of charge good batteries suffer, but it is probably small. Just keeping DRAM alive doesn't take a vast amount of power. This solution should be adequate to handle even Katrina-length power outages. Beyond that, disk corruption is unlikely to be your major concern.


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