Scattering of blobs from Avi:
This patchset converts the stack to scatter managed_bytes in lsa memory,
allowing large blobs (and collections) to be stored in memtable and cache.
Outside memtable/cache, they are still stored sequentially, but it is assumed
that the number of transient objects is bounded.
The approach taken here is to scatter managed_bytes data in multiple
blob_storage objects, but to linearize them back when accessing (for
example, to merge cells). This allows simple access through the normal
bytes_view. It causes an extra two copies, but copying a megabyte twice
is cheap compared to accessing a megabyte's worth of small cells, so
per-byte throughput is increased.
Testing show that lsa large object space is kept at zero, but throughput
is bad because Scylla easily overwhelms the disk with large blobs; we'll
need Glauber's throttling patches or a really fast disk to see good
throughput with this.
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