On Jan 30, 2009, at 7:44 AM, Jaime Magiera wrote:
I've been using NSFIleHandle for a project that inserts data into a
file and synchs it back to disk. Everything went smoothly until the
app started getting used for larger files ( > 200 megs). First, I
ran into the NSFileHandle -> NSData 256 megs conundrum. That was
solved by, as others suggested on this list in other threads,
iterating with readDataOfLength or availableData instead of a
single readDataToEndOfFile. That worked well. I've got pools set up
to keep the memory down. The problem now is that the reads are still
really slow. For example, with a read length of 50 megs, I can only
get in 3 reads per second. My apps ends up taking almost a minute to
perform all of its functions on a file of 500 megs.
Perhaps my overall approach was wrong to start out. What I've been
doing is opening a file handle, copying the data after the insertion
point to an NSData, truncating the file handle at the insertion
point, adding the new data, then adding back the trimmed data. This
works fairly well if the insert point is towards the end of the
file. However, there are instances where I need to insert a few
hundred kb into a the file at a location only a few hundred kb into
the file.
xxx ^ xxxxxxxxxxxxxxxxx
The time hit comes from copying the trim data to the NSData. Is
there a better way to do this with NSFileHandle? Is there a better
way to do this than NSFileHandle?
I suspect your bottleneck is the filesystem. To know for sure you
could try the raw C calls and see if it speeds up. In any case,
instead of doing a grow/shrink on the file, write to a temp file
instead then swap them. This way you could also do optimized batch-
writes. This alone could speed the process up immensely.
Notwithstanding, how do you recover if your app crashes after you
truncate the file?
HTH,
Keary Suska
Esoteritech, Inc.
"Demystifying technology for your home or business"
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