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A comment on Show HN: LevelDB outperforms others on a cheap phone (Microsoft ESE and SQLite). Parent comment

Re:Show HN: LevelDB outperforms others on a cheap phone (Mic... by mtanski on Friday March 25, 2016 @02:17PM (#11361716)
> LMDB already runs on iOS and Android. In my experience you will have issues on some Android devices though. For instance, I recently found out that on some Android devices writes to a mmap-ed file through the usual POSIX interfaces are not immediatly reflected on reads through the mmap, which means that on those devices you MUST use MDB_WRITEMAP.

How is that possible if they are using Linux and UBC (Unified Buffer Cache). It's unlikely they anybody would go undo that from the VM/FS layer.

The only think I can think off this behavior happening in a Linux app is if you're creating a private mapping... Private mappings are COW and a write to the global would cause a copy of the old data for you app. They'll be no sycing till you msync()

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Re:Show HN: LevelDB outperforms others on a cheap phone (Mic... by catwell on Friday March 25, 2016 @02:55PM (#11361937)
To be honest I am not entirely sure of my interpretation of the issue yet. What I know is that on those devices you sometimes can't read your writes with LMDB if it's not using MDB_WRITEMAP.

While trying to debug I ended up on the only question ever asked by Howard Chu (LMDB author) on Stack Overflow: http://stackoverflow.com/questions/7061910/when-does-an-o-sy...

I am still trying to figure out what exactly is happening here but it's not very easy given that I do not even have access to a rooted phone that exhibits the issue.

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Re:Show HN: LevelDB outperforms others on a cheap phone (Mic... by hyc_symas on Friday March 25, 2016 @07:41PM (#11363583)
It really depends on where you're storing your DB files. If you're using the /data partition it should Just Work. If you're using external storage, yes, android does a bunch of stupid tricks to implement its secure storage, and that interferes with mmap.

Here's the code that resulted from that Stack Overflow question: https://github.com/LMDB/lmdb/blob/mdb.master/libraries/liblm...

After 36 years of writing code, I have more answers than questions.

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