Account Rent and Storage

// Anchor automatically computes rent-exempt minimum.
// Manual calculation: (data_len + 128) bytes * lamports_per_byte_year * 2 years

const MARGINFI_POSITION_SIZE: u64 = 256;  // example account layout size
const LAMPORTS_PER_BYTE_YEAR: u64 = 3_480;
const RENT_EXEMPT_YEARS: u64 = 2;

let rent_lamports = (MARGINFI_POSITION_SIZE + 128)
    * LAMPORTS_PER_BYTE_YEAR
    * RENT_EXEMPT_YEARS;
// = (256 + 128) * 3_480 * 2 = 2,672,640 lamports = 0.00267 SOL

// Anchor pattern:
#[derive(Accounts)]
pub struct OpenPosition<'info> {
    #[account(
        init,
        payer = user,
        space = 8 + Position::INIT_SPACE,
        seeds = [b"position", user.key().as_ref()],
        bump,
    )]
    pub position: Account<'info, Position>,
    #[account(mut)]
    pub user: Signer<'info>,
    pub system_program: Program<'info, System>,
}

// When closing:
#[derive(Accounts)]
pub struct ClosePosition<'info> {
    #[account(mut, close = user)]  // close = user refunds rent to user
    pub position: Account<'info, Position>,
    #[account(mut)]
    pub user: Signer<'info>,
}