What is the "seek penalty" mentioned in stablebit drivepool changelog?

[forkicks]

In the changelog here (for the beta versions):
https://dl.covecube.com/DrivePoolWindows/beta/download/changes.txt
this is mentioned:
" 1537 * [D] [Issue #28834] Report no seek penalty for the pool if all of its pool parts are also reporting no seek penalty (see 'dpcmd list-pools'). "

What exactly is this "seek penalty" mentioned here?

[Shane]

A seek penalty (in the context of storage devices) refers to a storage device needing additional time to perform a seek (begin retrieving data), and the OS can request a storage device to report whether or not it has a seek penalty (or how big it is). Mechanical hard drives have a seek penalty - they need time for the platter(s) to spin and for the head(s) to move into position before they can begin accessing data. Solid state drives (and RAM drives) usually do not (or it's very small). You can get edge cases where a drive might not itself have a seek penalty but the path to it does (e.g. a SSD being virtually mounted over a slow network).

TLDR it can be a reasonably accurate indicator as to whether a device is a HDD or SSD (or at least close enough for practical purposes).

[forkicks]

Does it affect something in how drivepool works? File placement, directory traversal or lookup, etc?

I guess what I would like to know is what's the purpose of having that information for drivepool?

[Shane]

I believe it's used when DrivePool has a choice of drive(s) to pick; a seek penalty will influence which are chosen.

E.g. if you tell it to open a file (e.g. a movie) that's stored on more than one drive in the pool, it will look at which of those drives are busy and which have no penalty. So if your file was on a SSD and a HDD it would prefer to open it from the SSD unless the latter was already busy with something else.

The update means this now also applies to nested pools; in such situations (e.g. a pool consisting of a pool of SSDs and a pool of HDDs) the combined pool can now know which of it's comprising pools have no seek penalty.

[everydayz]

So, the "seek penalty" is basically a measure of how much it'll slow down your system when reading or writing data from/to the disk.