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

[Joshua Judson Rosen]

On 09/28/2017 02:14 PM, mark wrote:
> AWS/EBS is not LVM under the covers, it's more like NFS; and snapshots are more like VMware & how it does snapshots.

I have never used VMWare and have no idea how it does anything. Can you provide more insight on what that means?


> The OS cache exclusion refers to read-ahead and write caching going on in RAM.

Yes, I got that. The reason I included that in the citation was actually that I took it
as supporting my "this looks like atomic COW snapshotting" conclusion, because that's
exactly what I'm accustomed to getting through LVM (snapshotting a block device
captures all of the blocks that *have actually been written* at the time of the snapshot).

> On Sep 28, 2017 1:17 PM, "Joshua Judson Rosen" <rozzin at hackerposse.com <mailto:rozzin at hackerposse.com>> wrote:
> 
>     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/ <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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