Is Amazon AWS/EBS snapshotting just LVM, or what?

[Joshua Judson Rosen]

I'm working on a project that uses Amazon AWS-provided VPS instances,
and the other guy on the project is telling me that "snapshotting hourly may degrade performance",
and I'm trying to determine where that's actually true. My gut feeling is that it sounds kind of bogus.

>From the information I've been able to find about how Amazon's stuff works (either in terms
of how it's _implemented_ [for which I'm finding basically no insight] or how it's _characterized_
[in the engineering sense, not the literary sense]...), it really sounds a _lot_ like Amazon
is just using LVM snapshots, e.g. from <https://aws.amazon.com/ebs/faqs/>:

	"snapshots can be done in real time while the volume is attached and in use.
	 However, snapshots only capture data that has been written to your Amazon EBS volume,
	 which might exclude any data that has been locally cached by your application or OS."

	"By design, an EBS Snapshot of an entire 16 TB volume should take no longer than the time
	 it takes to snapshot an entire 1 TB volume. However, the actual time taken to create
	 a snapshot depends on several factors including the amount of data that has changed
	 since the last snapshot of the EBS volume."

... though I'm not entirely sure how to interpret that last bit about "time taken to create a snapshot
depends on... the amount of data that has changed since the last snapshot";
the _first half of that statement_ reads as "creating a snapshot is constant time",
which basically screams to me "copy-on-write just like LVM, and they're probably implemented
in terms of LVM".

Any insight here as to whether my gut is correct on this, or whether I'm actually likely
to notice an impact from hourly snapshots of, say, a 200-GB volume? How about a 1-TB volume?

The only thing I'm seeing from Amazon that seems to _vaguely_ support (maybe) the notion
that `snapshotting too often' would be something to worry about is this bit from elsewhere
in that same FAQ page (under the heading of "performance", whereas the others were
under the heading of "snapshots" and a subheading of "performance consistency of my HDD-backed volumes":

	Another factor is taking a snapshot which will decrease expected write performance
	down to the baseline rate, until the snapshot completes.

... and, taken in the context of the previously-cited notes about snapshots being
`not base on volume-size but maybe influenced by changed-since-last-snapshot set size'
(and in the context of the explanations they give for HDD-backed vs. SSD-backed storage),
I'm basically reading that as:

	`if you're using HDD-backed storage then it's because you care about *throughput*
	 more than *response time* and are likely to be monitoring throughput,
	 and if you're monitoring throughput you may notice a *momentary dip in throughput*
	 as the *HDDs* need to seek around to find the volume boundaries and set up the COW records.'

Even if you don't have any insight into what's actually happening under the covers at Amazon,
does my reading of all of this sound right to you?

And, perhaps more interestingly, are these same caveats from Amazon generally applicable to LVM?

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