On 2016-09-12 12:51, David Sterba wrote:
On Mon, Sep 12, 2016 at 10:54:40AM -0400, Austin S. Hemmelgarn wrote:
Somebody has put that table on the wiki, so it's a good starting point.
I'm not sure we can fit everything into one table, some combinations do
not bring new information and we'd need n-dimensional matrix to get the
whole picture.
Agreed, especially because some things are only bad in specific
circumstances (For example, snapshots generally work fine on almost
anything, until you get into the range of more than about 250, then they
start causing issues).
The performance aspect could be hard to estimate. Each feature has some
cost, we can document what's expected hit but various combinations and
actual runtime performance is unpredictable. I'd rather let the tools do
what the user asks for, as we might not be able to even detect there are
some bad external factors. I think that 250 snapshots would perform
better on an ssd than a rotational disk. In the end this leads to the
"dos & don'ts".
In general yes in this case, but performance starts to degrade
exponentially beyond a certain point. The difference between (for
example) 10 and 20 snapshots is not as much as between 1000 and 1010.
The problem here is that we don't really have a BCP document that anyone
ever reads. A lot of stuff that may seem obvious to us after years of
working with BTRFS isn't going to be to a newcomer, and it's a lot more
likely that some random person will get things write if we have a good,
central BCP document than if it stays as scattered tribal knowledge.
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