Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
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Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#2Memory-Mapped Files = access violations when a disk read fails. If you're not prepared to handle those, don't use memory-mapped files. (Access violation exceptions are the same thing that happens when you attempt to read a null pointer)
Then there's the part with writes being delayed. Be prepared to deal with blocks not necessarily updating to disk in the order they were written to, and 10 seconds after the fact. This can make power failures cause inconsistencies.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#3The TLDR is that MMAP sorta does what you want, but DBMSes need more control over how/when data is paged in/out of memory. Without this extra control, there can be issues with transactional safety, and performance.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#4I've become convinced that there are very few, if any, reasons to MMAP a file on disk. It seems to simplify things in the common case, but in the end it adds a massive amount of unnecessary complexity.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#5maybe a stupid question but what is wrong with coffee and spicy food?
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#6Old timers will recall when using mmap was a prominently promoted selling point for the “no sql” dbms.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#7maybe a stupid question but what is wrong with coffee and spicy food?
For the majority of the world, nothing. But if your diet consists of fairly bland food then it can result in unpleasant trips to the toilet.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#8Memory-Mapped Files = access violations when a disk read fails. If you're not prepared to handle those, don't use memory-mapped files. (Access violation exceptions are the same thing that happens when you attempt to read a null pointer) Then there's the part with writes being delayed. Be prepared to deal with blocks not necessarily updating to disk in the order they were written to, and 10 seconds after the fact. Thi…
> Be prepared to deal with blocks not necessarily updating to disk in the order they were written to, and 10 seconds after the fact. This can make power failures cause inconsistencies.
This is not specific to mmap -- regular old write() calls have the same behavior. You need to fsync() (or, with mmap, msync()) to guarantee data is on disk.
Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#9Re: Are You Sure You Want to Use MMAP in Your Database Management System? (2022)
#10Memory-Mapped Files = access violations when a disk read fails. If you're not prepared to handle those, don't use memory-mapped files. (Access violation exceptions are the same thing that happens when you attempt to read a null pointer) Then there's the part with writes being delayed. Be prepared to deal with blocks not necessarily updating to disk in the order they were written to, and 10 seconds after the fact. Thi…
does that get delivered as SIGSEGV to the process or something else?