Reserve storage usage up-front when downloading image layers to tmpfs

[zjs]

User Statement:

As a user, I do not want to unnecessarily run out of tmpfs storage space due to the download of image layers. As a user, I do not want to need to know the size of images or layers that will be used in a given VCH ahead of time.

Details:

Currently, this can happen because we download up to three layers concurrently, without any reservation. Reserving space will force downloads to occur sequentially when necessary, allowing layers to be combined to free up space before continuing the download process.

Acceptance Criteria:

• A test is added which downloads an image with several large layers that would have failed without fixing this issue.
• This issue is not intended to address pulling of a single layer that is larger than the available capacity, however it should provide a sane error in that circumstance (when reservation cannot succeed even when no other layers are in progress).

[renmaosheng]

Jun, with the storage quota. would this issue be mitigated? thanks.

[wjun]

@zjs I wonder the trade off between performance and failure/reset memory?