# Browser Storage Notes
Browsers examined: Firefox 67, Chrome 75
The examination below applies to the default, non-persistent storage policy.
## Quota Measurement
Browsers appear to enforce and measure the quota in terms of space on disk, not
data stored, so you may be able to store more data than the simple sum of all
input data depending on how compressible your data is.
## Quota Limit
Specs and documentation suggest we should consistently receive
`QuotaExceededError` when we're near space limits, but the reality is a bit
When we are low on disk space overall or near the group limit / origin quota:
* Log database may fail to start with AbortError
* IndexedDB fails to start for crypto: AbortError in connect from
* When near the quota, QuotaExceededError is used more consistently
* The first error will be QuotaExceededError
* Future write attempts will fail with various errors when space is low,
including nonsense like "InvalidStateError: A mutation operation was
attempted on a database that did not allow mutations."
* Once you start getting errors, the DB is effectively wedged in read-only
* Can revive access if you reopen the DB
## Cache Eviction
While the Storage Standard says all storage for an origin group should be
limited by a single quota, in practice, browsers appear to handle `localStorage`
separately from the others, so it has a separate quota limit and isn't evicted
when low on space.
* Chrome, Firefox
* IndexedDB for origin deleted
* Local Storage remains in place
## Persistent Storage
Storage Standard offers a `navigator.storage.persist` API that can be used to
request persistent storage that won't be deleted by the browser because of low
* Chrome 75 seems to grant this without any prompt based on [interaction
* Firefox 67 shows a prompt to grant
* Reverting persistent seems to require revoking permission _and_ clearing
## Storage Estimation
Storage Standard offers a `navigator.storage.estimate` API to get some clue of
how much space remains.
* Chrome, Firefox
* Can run this at any time to request an estimate of space remaining
* Returns `0` for `usage` if a site is persisted